Method, apparatus and storage medium for image encoding/decoding using subpicture

ABSTRACT

Disclosed herein are a method, an apparatus and a storage medium for performing encoding/decoding on an image using a subpicture. A picture may be partitioned into various units such as subpictures and slices, and various syntax elements and structures may be used to specify the units. In an embodiment, there is a method for configuring a reference picture list for motion compensation in the case where the location of a reference subpicture to be referred to for motion compensation is different from the location of a target subpicture. Further, in an embodiment, there is provided a method for providing the configuration of a reference picture list and an override on the number of activated reference indexes.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. patentapplication Ser. No. 17/955,278, filed Sep. 28, 2022, which is acontinuation application of U.S. patent application Ser. No. 17/221,635,filed Apr. 2, 2021, and claims priority under 35 U.S.C. § 119(a) toKorean application numbers 10-2020-0040765, filed Apr. 3, 2020 and10-2021-0041924, filed Mar. 31, 2021, in the Korean IntellectualProperty Office, which is incorporated herein by reference in itsentirety.

BACKGROUND OF THE INVENTION 1. Technical Field

The present disclose relates generally to a method, an apparatus and astorage medium for image encoding/decoding. More particularly, thepresent disclosure relates to a method, an apparatus and a storagemedium for performing encoding/decoding on an image using a subpicture.

2. Description of the Related Art

With the continuous development of the information and communicationindustries, broadcasting services supporting High-Definition (HD)resolution have been popularized all over the world. Through thispopularization, a large number of users have become accustomed tohigh-resolution and high-definition images and/or video.

To satisfy users' demand for high definition, many institutions haveaccelerated the development of next-generation imaging devices. Users'interest in UHD TVs, having resolution that is more than four times ashigh as that of Full HD (FHD) TVs, as well as High-Definition TVs (HDTV)and FHD TVs, has increased. As interest therein has increased, imageencoding/decoding technology for images having higher resolution andhigher definition is currently required.

As image compression technology, there are various technologies, such asinter-prediction technology, intra-prediction technology, transform,quantization technology, and entropy coding technology.

Inter-prediction technology is technology for predicting the value of apixel included in a current picture using a picture previous to and/or apicture subsequent to the current picture. Intra-prediction technologyis technology for predicting the value of a pixel included in a currentpicture using information about pixels in the current picture. Transformand quantization technology may be technology for compressing the energyof a residual signal. The entropy coding technology is technology forassigning a short codeword to a frequently occurring value and assigninga long codeword to a less frequently occurring value.

By utilizing this image compression technology, data about images may beeffectively compressed, transmitted, and stored.

SUMMARY OF THE INVENTION

An embodiment is intended to provide a method, an apparatus and astorage medium that perform encoding/decoding on an image using asubpicture.

An embodiment is intended to provide a method, an apparatus and astorage medium that configure a reference picture list for motioncompensation in the case where the location of a reference subpicture tobe referred to for motion compensation is different from the location ofa target subpicture.

An embodiment is intended to provide a method, an apparatus and astorage medium that configure a reference picture list and provide anoverride for the number of activated reference indexes.

In accordance with an aspect of the present invention to accomplish theabove object, there is provided a decoding method, including acquiringinformation about a target picture from a bitstream; and performingdecoding on the target picture using the information about the targetpicture, wherein the decoding includes a determination of a referencepicture list for the target picture and a prediction based on thereference picture list, and wherein the information about the targetpicture includes a slice header for a slice of the target picture.

The determination of the reference picture list in the slice header maybe performed when a Network Abstraction Layer (NAL) unit type of theslice is not a specific type.

The bitstream may include a picture header for the target picture.

The determination of the reference picture list in the slice header maybe performed when information related to the reference picture list isnot present in the picture header for the target picture.

The bitstream may include first information.

The decoding may include a determination of whether an override is to beperformed on a number of activated reference indexes in the sliceheader.

The determination of whether the override is to be performed may beperformed regardless of a value of the first information.

The first information may be information indicating whether referencepicture list syntax elements are capable of being present in sliceheaders of slices for which a NAL unit type is a specific type.

The bitstream may further include second information.

The override may include decoding on the second information.

The second information may be information indicating whether an overrideis to be performed on the number of activated reference indexes for thereference picture list.

When the second information indicates that the override is to beperformed, the information indicating the number of activated referenceindexes for the reference picture list may be decoded.

The specific type may be an Instantaneous Decoding Refresh (IDR)_N_LP orIDR_W_Random Access Decodable Leading (RADL) type.

In accordance with another aspect of the present invention to accomplishthe above object, there is provided an encoding method, includinggenerating information about a target picture by performing encoding onthe target picture; and generating a bitstream including the informationabout the target picture, wherein the information about the targetpicture includes a determination of a reference picture list for thetarget picture and information for a prediction based on the referencepicture list, and wherein the information about the target pictureincludes a slice header for a slice of the target picture.

The determination of the reference picture list in the slice header maybe performed when a Network Abstraction Layer (NAL) unit type of theslice is not a specific type.

The bitstream may include a picture header for the target picture.

The determination of the reference picture list in the slice header maybe performed when information related to the reference picture list isnot present in the picture header.

The bitstream may include first information.

The information about the target picture may further include informationrequired to determine whether an override is to be performed on a numberof activated reference indexes in the slice header.

The determination of whether the override is to be performed may beperformed regardless of a value of the first information.

The first information may be information indicating whether referencepicture list syntax elements are capable of being present in sliceheaders of slices for which a NAL unit type is a specific type.

The bitstream may further include second information.

The second information may be information indicating whether an overrideis to be performed on the number of activated reference indexes for thereference picture list.

When the second information indicates that the override is to beperformed, the information indicating the number of activated referenceindexes for the reference picture list may be encoded.

The specific type may be an Instantaneous Decoding Refresh (IDR)_N_LP orIDR_W_Random Access Decodable Leading (RADL) type.

In accordance with a further aspect of the present invention toaccomplish the above object, there is provided a computer-readablestorage medium storing a bitstream, wherein the bitstream includesinformation about a target picture, the information about the targetpicture is acquired from the bitstream, decoding on the target pictureis performed using the information about the target picture, thedecoding includes a determination of a reference picture list for thetarget picture and a prediction based on the reference picture list, andthe information about the target picture includes a slice header for aslice of the target picture.

The determination of the reference picture list in the slice header maybe performed when a NAL unit type of the slice is not a specific type.

The bitstream may include a picture header for the target picture.

The determination of the reference picture list in the slice header maybe performed when information related to the reference picture list isnot present in the picture header of the target picture.

The bitstream may include first information.

The decoding may include a determination of whether an override is to beperformed on a number of activated reference indexes in the sliceheader.

The determination of whether the override is to be performed may beperformed regardless of a value of the first information.

The first information may be information indicating whether referencepicture list syntax elements are capable of being present in sliceheaders of slices for which a NAL unit type is a specific type.

The bitstream may further include second information.

The override may include decoding on the second information.

The second information may be information indicating whether an overrideis to be performed on the number of activated reference indexes for thereference picture list.

When the second information indicates that the override is to beperformed, the information indicating the number of activated referenceindexes for the reference picture list may be decoded.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentdisclosure will be more clearly understood from the following detaileddescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a block diagram illustrating the configuration of anembodiment of an encoding apparatus to which the present disclosure isapplied;

FIG. 2 is a block diagram illustrating the configuration of anembodiment of a decoding apparatus to which the present disclosure isapplied;

FIG. 3 is a diagram schematically illustrating the partition structureof an image when the image is encoded and decoded;

FIG. 4 is a diagram illustrating the form of a Prediction Unit (PU) thata Coding Unit (CU) can include;

FIG. 5 is a diagram illustrating the form of a Transform Unit (TU) thatcan be included in a CU;

FIG. 6 illustrates splitting of a block according to an example;

FIG. 7 is a diagram for explaining an embodiment of an intra-predictionprocedure;

FIG. 8 is a diagram illustrating reference samples used in anintra-prediction procedure;

FIG. 9 is a diagram for explaining an embodiment of an inter-predictionprocedure;

FIG. 10 illustrates spatial candidates according to an embodiment;

FIG. 11 illustrates the order of addition of motion information ofspatial candidates to a merge list according to an embodiment;

FIG. 12 illustrates a transform and quantization process according to anexample;

FIG. 13 illustrates diagonal scanning according to an example;

FIG. 14 illustrates horizontal scanning according to an example;

FIG. 15 illustrates vertical scanning according to an example;

FIG. 16 is a configuration diagram of an encoding apparatus according toan embodiment;

FIG. 17 is a configuration diagram of a decoding apparatus according toan embodiment;

FIG. 18 illustrates the division of a picture in a raster-scan slicemode according to an example;

FIG. 19 illustrates the division of a picture in a rectangular slicemode according to an example;

FIG. 20 illustrates tiles and rectangular slices in a picture accordingto an example;

FIG. 21 illustrates the division of a picture into subpictures accordingto an example;

FIG. 22 illustrates code for subpicture ID mapping according to anexample;

FIG. 23 illustrates signaling of slice addresses in a raster-scan slicemode according to an example;

FIG. 24 illustrates signaling of slice addresses in a rectangular slicemode according to an example;

FIG. 25 illustrates a part of the structure of a sequence parameter setaccording to an example;

FIG. 26 illustrates a method for deriving a subpicture ID according toan example;

FIG. 27 illustrates a scheme for utilizing a slice index for derivingthe address of a CTB in a slice according to an example;

FIG. 28 illustrates a scheme for utilizing a slice index for deriving aCTB address in a target slice according to an example;

FIG. 29 illustrates a part of the structure of a picture parameter setaccording to an example;

FIG. 30 illustrates code for subpicture ID mapping according to anexample;

FIG. 31 illustrates a change in a subpicture ID according to an example;

FIG. 32 illustrates a part of the structure of a sequence parameter setindicating whether there is syntax related to a reference picture listaccording to an example;

FIG. 33 illustrates a part of the structure of a picture parameter setindicating whether there are mixed NAL unit types according to anexample;

FIG. 34 illustrates a slice header for configuring a reference picturelist according to an embodiment;

FIG. 35 illustrates the case where the locations of subpictures in apicture are changed according to an embodiment;

FIG. 36 illustrates the case where the locations of subpictures treatedas pictures are changed according to an embodiment;

FIG. 37 illustrates another part of the structure of a picture parameterset indicating whether there are mixed NAL unit types according to anexample;

FIG. 38 illustrates an additional case where the locations ofsubpictures in a picture are changed according to an embodiment;

FIG. 39 illustrates the case where the locations of subpictures treatedas pictures are changed according to an embodiment;

FIG. 40 illustrates a further part of the structure of a pictureparameter set indicating whether there are mixed NAL unit typesaccording to an example;

FIG. 41 is a flowchart of an encoding method according to an embodiment;and

FIG. 42 is a flowchart of a decoding method according to an embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention may be variously changed, and may have variousembodiments, and specific embodiments will be described in detail belowwith reference to the attached drawings. However, it should beunderstood that those embodiments are not intended to limit the presentinvention to specific disclosure forms, and that they include allchanges, equivalents or modifications included in the spirit and scopeof the present invention.

Detailed descriptions of the following exemplary embodiments will bemade with reference to the attached drawings illustrating specificembodiments. These embodiments are described so that those havingordinary knowledge in the technical field to which the presentdisclosure pertains can easily practice the embodiments. It should benoted that the various embodiments are different from each other, but donot need to be mutually exclusive of each other. For example, specificshapes, structures, and characteristics described here may beimplemented as other embodiments without departing from the spirit andscope of the embodiments in relation to an embodiment. Further, itshould be understood that the locations or arrangement of individualcomponents in each disclosed embodiment can be changed without departingfrom the spirit and scope of the embodiments. Therefore, theaccompanying detailed description is not intended to restrict the scopeof the disclosure, and the scope of the exemplary embodiments is limitedonly by the accompanying claims, along with equivalents thereof, as longas they are appropriately described.

In the drawings, similar reference numerals are used to designate thesame or similar functions in various aspects. The shapes, sizes, etc. ofcomponents in the drawings may be exaggerated to make the descriptionclear.

Terms such as “first” and “second” may be used to describe variouscomponents, but the components are not restricted by the terms. Theterms are used only to distinguish one component from another component.For example, a first component may be named a second component withoutdeparting from the scope of the present specification. Likewise, asecond component may be named a first component. The terms “and/or” mayinclude combinations of a plurality of related described items or any ofa plurality of related described items.

It will be understood that when a component is referred to as being“connected” or “coupled” to another component, the two components may bedirectly connected or coupled to each other, or intervening componentsmay be present between the two components. On the other hand, it will beunderstood that when a component is referred to as being “directlyconnected or coupled”, no intervening components are present between thetwo components.

Also, components described in the embodiments are independently shown inorder to indicate different characteristic functions, but this does notmean that each of the components is formed of a separate piece ofhardware or software. That is, the components are arranged and includedseparately for convenience of description. For example, at least two ofthe components may be integrated into a single component. Conversely,one component may be divided into multiple components. An embodimentinto which the components are integrated or an embodiment in which somecomponents are separated is included in the scope of the presentspecification as long as it does not depart from the essence of thepresent specification.

Further, in the exemplary embodiments, an expression describing that acomponent “comprises” a specific component means that additionalcomponents may be included within the scope of the practice or thetechnical spirit of exemplary embodiments, but does not preclude thepresence of components other than the specific component.

The terms used in the present specification are merely used to describespecific embodiments and are not intended to limit the presentinvention. A singular expression includes a plural expression unless adescription to the contrary is specifically pointed out in context. Inthe present specification, it should be understood that the terms suchas “include” or “have” are merely intended to indicate that features,numbers, steps, operations, components, parts, or combinations thereofare present, and are not intended to exclude the possibility that one ormore other features, numbers, steps, operations, components, parts, orcombinations thereof will be present or added. That is, in the presentinvention, an expression describing that a component “comprises” aspecific component means that additional components may be includedwithin the scope of the practice of the present invention or thetechnical spirit of the present invention, but does not preclude thepresence of components other than the specific component.

Some components of the present invention are not essential componentsfor performing essential functions, but may be optional components forimproving only performance. The embodiments may be implemented usingonly essential components for implementing the essence of theembodiments. For example, a structure including only essentialcomponents, excluding optional components used only to improveperformance, is also included in the scope of the embodiments.

Embodiments will be described in detail below with reference to theaccompanying drawings so that those having ordinary knowledge in thetechnical field to which the embodiments pertain can easily practice theembodiments. In the following description of the embodiments, detaileddescriptions of known functions or configurations which are deemed tomake the gist of the present specification obscure will be omitted.Further, the same reference numerals are used to designate the samecomponents throughout the drawings, and repeated descriptions of thesame components will be omitted.

Hereinafter, “image” may mean a single picture constituting a video, ormay mean the video itself. For example, “encoding and/or decoding of animage” may mean “encoding and/or decoding of a video”, and may also mean“encoding and/or decoding of any one of images constituting the video”.

Hereinafter, the terms “video” and “motion picture” may be used to havethe same meaning, and may be used interchangeably with each other.

Hereinafter, a target image may be an encoding target image, which isthe target to be encoded, and/or a decoding target image, which is thetarget to be decoded. Further, the target image may be an input imagethat is input to an encoding apparatus or an input image that is inputto a decoding apparatus. And, a target image may be a current image,that is, the target to be currently encoded and/or decoded. For example,the terms “target image” and “current image” may be used to have thesame meaning, and may be used interchangeably with each other.

Hereinafter, the terms “image”, “picture”, “frame”, and “screen” may beused to have the same meaning and may be used interchangeably with eachother.

Hereinafter, a target block may be an encoding target block, i.e. thetarget to be encoded and/or a decoding target block, i.e. the target tobe decoded. Further, the target block may be a current block, i.e. thetarget to be currently encoded and/or decoded. Here, the terms “targetblock” and “current block” may be used to have the same meaning, and maybe used interchangeably with each other. A current block may denote anencoding target block, which is the target of encoding, during encodingand/or a decoding target block, which is the target of decoding, duringdecoding. Also, the current block may be at least one of a coding block,a prediction block, a residual block, and a transform block.

Hereinafter, the terms “block” and “unit” may be used to have the samemeaning, and may be used interchangeably with each other. Alternatively,“block” may denote a specific unit.

Hereinafter, the terms “region” and “segment” may be usedinterchangeably with each other.

In the following embodiments, specific information, data, a flag, anindex, an element, and an attribute may have their respective values. Avalue of “0” corresponding to each of the information, data, flag,index, element, and attribute may indicate a false, a logical false or afirst predefined value. In other words, the value of “0”, a false,logical false, and a first predefined value may be used interchangeablywith each other. A value of “1” corresponding to each of theinformation, data, flag, index, element, and attribute may indicate atrue, a logical true or a second predefined value. In other words, thevalue of “1”, true, logical true, and a second predefined value may beused interchangeably with each other.

When a variable such as i or j is used to indicate a row, a column, oran index, the value of i may be an integer of 0 or more or an integer of1 or more. In other words, in the embodiments, each of a row, a column,and an index may be counted from 0 or may be counted from 1.

In embodiments, the term “one or more” or the term “at least one” maymean the term “plural”. The term “one or more” or the term “at leastone” may be used interchangeably with “plural”.

Below, the terms to be used in embodiments will be described.

Encoder: An encoder denotes a device for performing encoding. That is,an encoder may mean an encoding apparatus.

Decoder: A decoder denotes a device for performing decoding. That is, adecoder may mean a decoding apparatus.

Unit: A unit may denote the unit of image encoding and decoding. Theterms “unit” and “block” may be used to have the same meaning, and maybe used interchangeably with each other.

-   -   A unit may be an M×N array of samples. Each of M and N may be a        positive integer. A unit may typically mean an array of samples        in the form of two-dimensions.    -   In the encoding and decoding of an image, “unit” may be an area        generated by the partitioning of one image. In other words,        “unit” may be a region specified in one image. A single image        may be partitioned into multiple units. Alternatively, one image        may be partitioned into sub-parts, and the unit may denote each        partitioned sub-part when encoding or decoding is performed on        the partitioned sub-part.    -   In the encoding and decoding of an image, predefined processing        may be performed on each unit depending on the type of the unit.    -   Depending on functions, the unit types may be classified into a        macro unit, a Coding Unit (CU), a Prediction Unit (PU), a        residual unit, a Transform Unit (TU), etc. Alternatively,        depending on functions, the unit may denote a block, a        macroblock, a coding tree unit, a coding tree block, a coding        unit, a coding block, a prediction unit, a prediction block, a        residual unit, a residual block, a transform unit, a transform        block, etc. For example, a target unit, which is the target of        encoding and/or decoding, may be at least one of a CU, a PU, a        residual unit, and a TU.    -   The term “unit” may mean information including a luminance        (luma) component block, a chrominance (chroma) component block        corresponding thereto, and syntax elements for respective blocks        so that the unit is designated to be distinguished from a block.    -   The size and shape of a unit may be variously implemented.        Further, a unit may have any of various sizes and shapes. In        particular, the shapes of the unit may include not only a        square, but also a geometric figure that can be represented in        two dimensions (2D), such as a rectangle, a trapezoid, a        triangle, and a pentagon.    -   Further, unit information may include one or more of the type of        a unit, the size of a unit, the depth of a unit, the order of        encoding of a unit and the order of decoding of a unit, etc. For        example, the type of a unit may indicate one of a CU, a PU, a        residual unit and a TU.    -   One unit may be partitioned into sub-units, each having a        smaller size than that of the relevant unit.

Depth: A depth may mean an extent to which the unit is partitioned.Further, the depth of the unit may indicate the level at which thecorresponding unit is present when unit(s) are represented by a treestructure.

-   -   Unit partition information may include a depth indicating the        depth of a unit. A depth may indicate the number of times the        unit is partitioned and/or the degree to which the unit is        partitioned.    -   In a tree structure, it may be considered that the depth of a        root node is the smallest, and the depth of a leaf node is the        largest. The root node may be the highest (top) node. The leaf        node may be a lowest node.    -   A single unit may be hierarchically partitioned into multiple        sub-units while having depth information based on a tree        structure. In other words, the unit and sub-units, generated by        partitioning the unit, may correspond to a node and child nodes        of the node, respectively. Each of the partitioned sub-units may        have a unit depth. Since the depth indicates the number of times        the unit is partitioned and/or the degree to which the unit is        partitioned, the partition information of the sub-units may        include information about the sizes of the sub-units.    -   In a tree structure, the top node may correspond to the initial        node before partitioning. The top node may be referred to as a        “root node”. Further, the root node may have a minimum depth        value. Here, the top node may have a depth of level ‘0’.    -   A node having a depth of level ‘1’ may denote a unit generated        when the initial unit is partitioned once. A node having a depth        of level ‘2’ may denote a unit generated when the initial unit        is partitioned twice.    -   A leaf node having a depth of level ‘n’ may denote a unit        generated when the initial unit has been partitioned n times.    -   The leaf node may be a bottom node, which cannot be partitioned        any further. The depth of the leaf node may be the maximum        level. For example, a predefined value for the maximum level may        be 3.    -   A QT depth may denote a depth for a quad-partitioning. A BT        depth may denote a depth for a binary-partitioning. A TT depth        may denote a depth for a ternary-partitioning.

Sample: A sample may be a base unit constituting a block. A sample maybe represented by values from 0 to 2^(Bd)−1 depending on the bit depth(Bd).

-   -   A sample may be a pixel or a pixel value.    -   Hereinafter, the terms “pixel” and “sample” may be used to have        the same meaning, and may be used interchangeably with each        other.

A Coding Tree Unit (CTU): A CTU may be composed of a single lumacomponent (Y) coding tree block and two chroma component (Cb, Cr) codingtree blocks related to the luma component coding tree block. Further, aCTU may mean information including the above blocks and a syntax elementfor each of the blocks.

-   -   Each coding tree unit (CTU) may be partitioned using one or more        partitioning methods, such as a quad tree (QT), a binary tree        (BT), and a ternary tree (TT) so as to configure sub-units, such        as a coding unit, a prediction unit, and a transform unit. A        quad tree may mean a quarternary tree. Further, each coding tree        unit may be partitioned using a multitype tree (MTT) using one        or more partitioning methods.    -   “CTU” may be used as a term designating a pixel block, which is        a processing unit in an image-decoding and encoding process, as        in the case of partitioning of an input image.

Coding Tree Block (CTB): “CTB” may be used as a term designating any oneof a Y coding tree block, a Cb coding tree block, and a Cr coding treeblock.

Neighbor block: A neighbor block (or neighboring block) may mean a blockadjacent to a target block. A neighbor block may mean a reconstructedneighbor block.

Hereinafter, the terms “neighbor block” and “adjacent block” may be usedto have the same meaning and may be used interchangeably with eachother.

A neighbor block may mean a reconstructed neighbor block.

Spatial neighbor block; A spatial neighbor block may a block spatiallyadjacent to a target block. A neighbor block may include a spatialneighbor block.

-   -   The target block and the spatial neighbor block may be included        in a target picture.    -   The spatial neighbor block may mean a block, the boundary of        which is in contact with the target block, or a block located        within a predetermined distance from the target block.    -   The spatial neighbor block may mean a block adjacent to the        vertex of the target block. Here, the block adjacent to the        vertex of the target block may mean a block vertically adjacent        to a neighbor block which is horizontally adjacent to the target        block or a block horizontally adjacent to a neighbor block which        is vertically adjacent to the target block.

Temporal neighbor block: A temporal neighbor block may be a blocktemporally adjacent to a target block. A neighbor block may include atemporal neighbor block.

-   -   The temporal neighbor block may include a co-located block (col        block).    -   The col block may be a block in a previously reconstructed        co-located picture (col picture). The location of the col block        in the col-picture may correspond to the location of the target        block in a target picture. Alternatively, the location of the        col block in the col-picture may be equal to the location of the        target block in the target picture. The col picture may be a        picture included in a reference picture list.    -   The temporal neighbor block may be a block temporally adjacent        to a spatial neighbor block of a target block.

Prediction mode: The prediction mode may be information indicating themode used for intra prediction, or the mode used for inter prediction.

Prediction unit: A prediction unit may be a base unit for prediction,such as inter prediction, intra prediction, inter compensation, intracompensation, and motion compensation.

-   -   A single prediction unit may be divided into multiple partitions        having smaller sizes or sub-prediction units. The multiple        partitions may also be base units in the performance of        prediction or compensation. The partitions generated by dividing        the prediction unit may also be prediction units.

Prediction unit partition: A prediction unit partition may be the shapeinto which a prediction unit is divided.

Reconstructed neighbor unit: A reconstructed neighbor unit may be a unitwhich has already been decoded and reconstructed neighboring a targetunit.

-   -   A reconstructed neighbor unit may be a unit that is spatially        adjacent to the target unit or that is temporally adjacent to        the target unit.    -   A reconstructed spatial neighbor unit may be a unit which is        included in a target picture and which has already been        reconstructed through encoding and/or decoding.    -   A reconstructed temporal neighbor unit may be a unit which is        included in a reference image and which has already been        reconstructed through encoding and/or decoding. The location of        the reconstructed temporal neighbor unit in the reference image        may be identical to that of the target unit in the target        picture, or may correspond to the location of the target unit in        the target picture. Also, a reconstructed temporal neighbor unit        may be a block neighboring the corresponding block in a        reference image. Here, the location of the corresponding block        in the reference image may correspond to the location of the        target block in the target image. Here, the fact that the        locations of blocks correspond to each other may mean that the        locations of the blocks are identical to each other, may mean        that one block is included in another block, or may mean that        one block occupies a specific location in another block.

Subpicture: A picture may be divided into one or more subpictures. Asubpicture may be composed of one or more tile rows and one or more tilecolumns.

-   -   A subpicture may be a region having a rectangular shape in a        picture. Further, a subpicture may include one or more CTUs.    -   A subpicture may be a rectangular region of one or more slices        in a picture.    -   One subpicture may include one or more tiles, one or more        bricks, and/or one or more slices. One or more slices may        collectively cover a rectangular region of a picture.

Tile: A tile may be a region having a square or rectangular (i.e., anon-square rectangular) shape in a picture.

-   -   A picture may be partitioned into one or more tiles.    -   A tile may include one or more CTUs.    -   A tile may be partitioned into one or more bricks.

Brick: A brick may denote one or more CTU rows in a tile.

-   -   A tile may be partitioned into one or more bricks. Each brick        may include one or more CTU rows.    -   A tile that is not partitioned into two parts may also denote a        brick.

Slice: A slice may include one or more tiles in a picture.Alternatively, a slice may include one or more bricks in a tile.

-   -   A subpicture may contain one or more slices that collectively        cover a rectangular region of a picture. Consequently, each        subpicture boundary is also always a slice boundary, and each        vertical subpicture boundary is always also a vertical tile        boundary.

Parameter set: A parameter set may correspond to header information inthe internal structure of a bitstream.

A parameter set may include at least one of a video parameter set (VPS),a sequence parameter set (SPS), a picture parameter set (PPS), anadaptation parameter set (APS), a decoding parameter set (DPS), etc.

-   -   Information signaled through each parameter set may be applied        to pictures which refer to the corresponding parameter set. For        example, information in a VPS may be applied to pictures which        refer to the VPS. Information in an SPS may be applied to        pictures which refer to the SPS. Information in a PPS may be        applied to pictures which refer to the PPS.    -   Each parameter set may refer to a higher parameter set. For        example, a PPS may refer to an SPS. An SPS may refer to a VPS.    -   Further, a parameter set may include a tile group, slice header        information, and tile header information. The tile group may be        a group including multiple tiles. Also, the meaning of “tile        group” may be identical to that of “slice”.

Rate-distortion optimization: An encoding apparatus may userate-distortion optimization so as to provide high coding efficiency byutilizing combinations of the size of a coding unit (CU), a predictionmode, the size of a prediction unit (PU), motion information, and thesize of a transform unit (TU).

-   -   A rate-distortion optimization scheme may calculate        rate-distortion costs of respective combinations so as to select        an optimal combination from among the combinations. The        rate-distortion costs may be calculated using the equation        “D+λ*R”. Generally, a combination enabling the rate-distortion        cost to be minimized may be selected as the optimal combination        in the rate-distortion optimization scheme.    -   D may denote distortion. D may be the mean of squares of        differences (i.e. mean square error) between original transform        coefficients and reconstructed transform coefficients in a        transform unit.    -   R may denote the rate, which may denote a bit rate using        related-context information.    -   k denotes a Lagrangian multiplier. R may include not only coding        parameter information, such as a prediction mode, motion        information, and a coded block flag, but also bits generated due        to the encoding of transform coefficients.    -   An encoding apparatus may perform procedures, such as inter        prediction and/or intra prediction, transform, quantization,        entropy encoding, inverse quantization (dequantization), and/or        inverse transform so as to calculate precise D and R. These        procedures may greatly increase the complexity of the encoding        apparatus.    -   Bitstream: A bitstream may denote a stream of bits including        encoded image information.

Parsing: Parsing may be the decision on the value of a syntax element,made by performing entropy decoding on a bitstream. Alternatively, theterm “parsing” may mean such entropy decoding itself.

Symbol: A symbol may be at least one of the syntax element, the codingparameter, and the transform coefficient of an encoding target unitand/or a decoding target unit. Further, a symbol may be the target ofentropy encoding or the result of entropy decoding.

Reference picture: A reference picture may be an image referred to by aunit so as to perform inter prediction or motion compensation.Alternatively, a reference picture may be an image including a referenceunit referred to by a target unit so as to perform inter prediction ormotion compensation.

Hereinafter, the terms “reference picture” and “reference image” may beused to have the same meaning, and may be used interchangeably with eachother.

Reference picture list: A reference picture list may be a list includingone or more reference images used for inter prediction or motioncompensation.

-   -   The types of a reference picture list may include List Combined        (LC), List 0 (L0), List 1 (L1), List 2 (L2), List 3 (L3), etc.    -   For inter prediction, one or more reference picture lists may be        used.

Inter-prediction indicator: An inter-prediction indicator may indicatethe inter-prediction direction for a target unit. Inter prediction maybe one of unidirectional prediction and bidirectional prediction.Alternatively, the inter-prediction indicator may denote the number ofreference pictures used to generate a prediction unit of a target unit.Alternatively, the inter-prediction indicator may denote the number ofprediction blocks used for inter prediction or motion compensation of atarget unit.

Prediction list utilization flag: A prediction list utilization flag mayindicate whether a prediction unit is generated using at least onereference picture in a specific reference picture list.

An inter-prediction indicator may be derived using the prediction listutilization flag. In contrast, the prediction list utilization flag maybe derived using the inter-prediction indicator. For example, the casewhere the prediction list utilization flag indicates “0”, which is afirst value, may indicate that, for a target unit, a prediction block isnot generated using a reference picture in a reference picture list. Thecase where the prediction list utilization flag indicates “1”, which isa second value, may indicate that, for a target unit, a prediction unitis generated using the reference picture list.

Reference picture index: A reference picture index may be an indexindicating a specific reference picture in a reference picture list.

Picture Order Count (POC): A POC value for a picture may denote an orderin which the corresponding picture is displayed.

Motion vector (MV): A motion vector may be a 2D vector used for interprediction or motion compensation. A motion vector may mean an offsetbetween a target image and a reference image.

-   -   For example, a MV may be represented in a form such as (mv_(x),        mv_(y)). mv_(x) may indicate a horizontal component, and mv_(y)        may indicate a vertical component.    -   Search range: A search range may be a 2D area in which a search        for a MV is performed during inter prediction. For example, the        size of the search range may be M×N. M and N may be respective        positive integers.

Motion vector candidate: A motion vector candidate may be a block thatis a prediction candidate or the motion vector of the block that is aprediction candidate when a motion vector is predicted.

-   -   A motion vector candidate may be included in a motion vector        candidate list.

Motion vector candidate list: A motion vector candidate list may be alist configured using one or more motion vector candidates.

Motion vector candidate index: A motion vector candidate index may be anindicator for indicating a motion vector candidate in the motion vectorcandidate list. Alternatively, a motion vector candidate index may bethe index of a motion vector predictor.

Motion information: Motion information may be information including atleast one of a reference picture list, a reference image, a motionvector candidate, a motion vector candidate index, a merge candidate,and a merge index, as well as a motion vector, a reference pictureindex, and an inter-prediction indicator.

Merge candidate list: A merge candidate list may be a list configuredusing one or more merge candidates.

Merge candidate: A merge candidate may be a spatial merge candidate, atemporal merge candidate, a combined merge candidate, a combinedbi-prediction merge candidate, a candidate based on a history, acandidate based on an average of two candidates, a zero-merge candidate,etc. A merge candidate may include an inter-prediction indicator, andmay include motion information such as prediction type information, areference picture index for each list, a motion vector, a predictionlist utilization flag, and an inter-prediction indicator.

Merge index: A merge index may be an indicator for indicating a mergecandidate in a merge candidate list.

-   -   A merge index may indicate a reconstructed unit used to derive a        merge candidate between a reconstructed unit spatially adjacent        to a target unit and a reconstructed unit temporally adjacent to        the target unit.    -   A merge index may indicate at least one of pieces of motion        information of a merge candidate.

Transform unit: A transform unit may be the base unit of residual signalencoding and/or residual signal decoding, such as transform, inversetransform, quantization, dequantization, transform coefficient encoding,and transform coefficient decoding. A single transform unit may bepartitioned into multiple sub-transform units having a smaller size.Here, a transform may include one or more of a primary transform and asecondary transform, and an inverse transform may include one or more ofa primary inverse transform and a secondary inverse transform.

Scaling: Scaling may denote a procedure for multiplying a factor by atransform coefficient level.

-   -   As a result of scaling of the transform coefficient level, a        transform coefficient may be generated. Scaling may also be        referred to as “dequantization”.

Quantization Parameter (QP): A quantization parameter may be a valueused to generate a transform coefficient level for a transformcoefficient in quantization. Alternatively, a quantization parameter mayalso be a value used to generate a transform coefficient by scaling thetransform coefficient level in dequantization. Alternatively, aquantization parameter may be a value mapped to a quantization stepsize.

Delta quantization parameter: A delta quantization parameter may mean adifference value between a predicted quantization parameter and thequantization parameter of a target unit.

Scan: Scan may denote a method for aligning the order of coefficients ina unit, a block or a matrix. For example, a method for aligning a 2Darray in the form of a one-dimensional (1D) array may be referred to asa “scan”. Alternatively, a method for aligning a 1D array in the form ofa 2D array may also be referred to as a “scan” or an “inverse scan”.

Transform coefficient: A transform coefficient may be a coefficientvalue generated as an encoding apparatus performs a transform.Alternatively, the transform coefficient may be a coefficient valuegenerated as a decoding apparatus performs at least one of entropydecoding and dequantization.

-   -   A quantized level or a quantized transform coefficient level        generated by applying quantization to a transform coefficient or        a residual signal may also be included in the meaning of the        term “transform coefficient”.

Quantized level: A quantized level may be a value generated as theencoding apparatus performs quantization on a transform coefficient or aresidual signal. Alternatively, the quantized level may be a value thatis the target of dequantization as the decoding apparatus performsdequantization.

-   -   A quantized transform coefficient level, which is the result of        transform and quantization, may also be included in the meaning        of a quantized level.

Non-zero transform coefficient: A non-zero transform coefficient may bea transform coefficient having a value other than 0 or a transformcoefficient level having a value other than 0. Alternatively, a non-zerotransform coefficient may be a transform coefficient, the magnitude ofthe value of which is not 0, or a transform coefficient level, themagnitude of the value of which is not 0.

Quantization matrix: A quantization matrix may be a matrix used in aquantization procedure or a dequantization procedure so as to improvethe subjective image quality or objective image quality of an image. Aquantization matrix may also be referred to as a “scaling list”.

Quantization matrix coefficient: A quantization matrix coefficient maybe each element in a quantization matrix. A quantization matrixcoefficient may also be referred to as a “matrix coefficient”.

Default matrix: A default matrix may be a quantization matrix predefinedby the encoding apparatus and the decoding apparatus.

Non-default matrix: A non-default matrix may be a quantization matrixthat is not predefined by the encoding apparatus and the decodingapparatus. The non-default matrix may mean a quantization matrix to besignaled from the encoding apparatus to the decoding apparatus by auser.

Most Probable Mode (MPM): An MPM may denote an intra-prediction modehaving a high probability of being used for intra prediction for atarget block.

An encoding apparatus and a decoding apparatus may determine one or moreMPMs based on coding parameters related to the target block and theattributes of entities related to the target block.

The encoding apparatus and the decoding apparatus may determine one ormore MPMs based on the intra-prediction mode of a reference block. Thereference block may include multiple reference blocks. The multiplereference blocks may include spatial neighbor blocks adjacent to theleft of the target block and spatial neighbor blocks adjacent to the topof the target block. In other words, depending on which intra-predictionmodes have been used for the reference blocks, one or more differentMPMs may be determined.

-   -   The one or more MPMs may be determined in the same manner both        in the encoding apparatus and in the decoding apparatus. That        is, the encoding apparatus and the decoding apparatus may share        the same MPM list including one or more MPMs.

MPM list: An MPM list may be a list including one or more MPMs. Thenumber of the one or more MPMs in the MPM list may be defined inadvance.

MPM indicator: An MPM indicator may indicate an MPM to be used for intraprediction for a target block among one or more MPMs in the MPM list.For example, the MPM indicator may be an index for the MPM list.

-   -   Since the MPM list is determined in the same manner both in the        encoding apparatus and in the decoding apparatus, there may be        no need to transmit the MPM list itself from the encoding        apparatus to the decoding apparatus.    -   The MPM indicator may be signaled from the encoding apparatus to        the decoding apparatus. As the MPM indicator is signaled, the        decoding apparatus may determine the MPM to be used for intra        prediction for the target block among the MPMs in the MPM list.

MPM use indicator: An MPM use indicator may indicate whether an MPMusage mode is to be used for prediction for a target block. The MPMusage mode may be a mode in which the MPM to be used for intraprediction for the target block is determined using the MPM list.

-   -   The MPM use indicator may be signaled from the encoding        apparatus to the decoding apparatus.

Signaling: “signaling” may denote that information is transferred froman encoding apparatus to a decoding apparatus. Alternatively,“signaling” may mean information is included in in a bitstream or arecoding medium by a encoding apparatus. Information signaled by anencoding apparatus may be used by a decoding apparatus.

-   -   The encoding apparatus may generate encoded information by        performing encoding on information to be signaled. The encoded        information may be transmitted from the encoding apparatus to        the decoding apparatus. The decoding apparatus may obtain        information by decoding the transmitted encoded information.        Here, the encoding may be entropy encoding, and the decoding may        be entropy decoding.

Selective Signaling: Information may be signaled selectively. Aselective signaling FOR information may mean that an encoding apparatusselectively includes information (according to a specific condition) ina bitstream or a recording medium. Selective signaling for informationmay mean that a decoding apparatus selectively extracts information froma bitstream (according to a specific condition).

Omission of signaling: Signaling for information may be omitted.Omission of signaling for information on information may mean that anencoding apparatus does not include information (according to a specificcondition) in a bitstream or a recording medium. Omission of signalingfor information may mean that a decoding apparatus does not extractinformation from a bitstream (according to a specific condition).

Statistic value: A variable, a coding parameter, a constant, etc. mayhave values that can be calculated. The statistic value may be a valuegenerated by performing calculations (operations) on the values ofspecified targets. For example, the statistic value may indicate one ormore of the average, weighted average, weighted sum, minimum value,maximum value, mode, median value, and interpolated value of the valuesof a specific variable, a specific coding parameter, a specificconstant, or the like.

FIG. 1 is a block diagram illustrating the configuration of anembodiment of an encoding apparatus to which the present disclosure isapplied.

An encoding apparatus 100 may be an encoder, a video encoding apparatusor an image encoding apparatus. A video may include one or more images(pictures). The encoding apparatus 100 may sequentially encode one ormore images of the video.

An encoding apparatus may generate encoded information by encodinginformation to be signaled. The encoded information may be transmittedfrom the encoding apparatus to a decoding apparatus. The decodingapparatus may acquire information by decoding the received encodedinformation. Here, encoding may be entropy encoding, and decoding may beentropy decoding.

Referring to FIG. 1 , the encoding apparatus 100 includes aninter-prediction unit 110, an intra-prediction unit 120, a switch 115, asubtractor 125, a transform unit 130, a quantization unit 140, anentropy encoding unit 150, a dequantization (inverse quantization) unit160, an inverse transform unit 170, an adder 175, a filter unit 180, anda reference picture buffer 190.

The encoding apparatus 100 may perform encoding on a target image usingan intra mode and/or an inter mode. In other words, a prediction modefor a target block may be one of an intra mode and an inter mode.

Hereinafter, the terms “intra mode”, “intra-prediction mode”,“intra-picture mode” and “intra-picture prediction mode” may be used tohave the same meaning, and may be used interchangeably with each other.

Hereinafter, the terms “inter mode”, “inter-prediction mode”,“inter-picture mode” and “inter-picture prediction mode” may be used tohave the same meaning, and may be used interchangeably with each other.

Hereinafter, the term “image” may indicate only part of an image, or mayindicate a block. Also, the processing of an “image” may indicatesequential processing of multiple blocks.

Further, the encoding apparatus 100 may generate a bitstream, includingencoded information, via encoding on the target image, and may outputand store the generated bitstream. The generated bitstream may be storedin a computer-readable storage medium and may be streamed through awired and/or wireless transmission medium.

When the intra mode is used as a prediction mode, the switch 115 mayswitch to the intra mode. When the inter mode is used as a predictionmode, the switch 115 may switch to the inter mode.

The encoding apparatus 100 may generate a prediction block of a targetblock. Further, after the prediction block has been generated, theencoding apparatus 100 may encode a residual block for the target blockusing a residual between the target block and the prediction block.

When the prediction mode is the intra mode, the intra-prediction unit120 may use pixels of previously encoded/decoded neighbor blocksadjacent to the target block as reference samples. The intra-predictionunit 120 may perform spatial prediction on the target block using thereference samples, and may generate prediction samples for the targetblock via spatial prediction. the prediction samples may mean samples inthe prediction block.

The inter-prediction unit 110 may include a motion prediction unit and amotion compensation unit.

When the prediction mode is an inter mode, the motion prediction unitmay search a reference image for the area most closely matching thetarget block in a motion prediction procedure, and may derive a motionvector for the target block and the found area based on the found area.Here, the motion-prediction unit may use a search range as a target areafor searching.

The reference image may be stored in the reference picture buffer 190.More specifically, an encoded and/or decoded reference image may bestored in the reference picture buffer 190 when the encoding and/ordecoding of the reference image have been processed.

Since a decoded picture is stored, the reference picture buffer 190 maybe a Decoded Picture Buffer (DPB).

The motion compensation unit may generate a prediction block for thetarget block by performing motion compensation using a motion vector.Here, the motion vector may be a two-dimensional (2D) vector used forinter-prediction. Further, the motion vector may indicate an offsetbetween the target image and the reference image.

The motion prediction unit and the motion compensation unit may generatea prediction block by applying an interpolation filter to a partial areaof a reference image when the motion vector has a value other than aninteger. In order to perform inter prediction or motion compensation, itmay be determined which one of a skip mode, a merge mode, an advancedmotion vector prediction (AMVP) mode, and a current picture referencemode corresponds to a method for predicting the motion of a PU includedin a CU, based on the CU, and compensating for the motion, and interprediction or motion compensation may be performed depending on themode.

The subtractor 125 may generate a residual block, which is thedifferential between the target block and the prediction block. Aresidual block may also be referred to as a “residual signal”.

The residual signal may be the difference between an original signal anda prediction signal. Alternatively, the residual signal may be a signalgenerated by transforming or quantizing the difference between anoriginal signal and a prediction signal or by transforming andquantizing the difference. A residual block may be a residual signal fora block unit.

The transform unit 130 may generate a transform coefficient bytransforming the residual block, and may output the generated transformcoefficient. Here, the transform coefficient may be a coefficient valuegenerated by transforming the residual block.

The transform unit 130 may use one of multiple predefined transformmethods when performing a transform.

The multiple predefined transform methods may include a Discrete CosineTransform (DCT), a Discrete Sine Transform (DST), a Karhunen-LoeveTransform (KLT), etc.

The transform method used to transform a residual block may bedetermined depending on at least one of coding parameters for a targetblock and/or a neighbor block. For example, the transform method may bedetermined based on at least one of an inter-prediction mode for a PU,an intra-prediction mode for a PU, the size of a TU, and the shape of aTU. Alternatively, transformation information indicating the transformmethod may be signaled from the encoding apparatus 100 to the decodingapparatus 200.

When a transform skip mode is used, the transform unit 130 may omittransforming the residual block.

By applying quantization to the transform coefficient, a quantizedtransform coefficient level or a quantized level may be generated.Hereinafter, in the embodiments, each of the quantized transformcoefficient level and the quantized level may also be referred to as a‘transform coefficient’.

The quantization unit 140 may generate a quantized transform coefficientlevel (i.e., a quantized level or a quantized coefficient) by quantizingthe transform coefficient depending on quantization parameters. Thequantization unit 140 may output the quantized transform coefficientlevel that is generated. In this case, the quantization unit 140 mayquantize the transform coefficient using a quantization matrix.

The entropy encoding unit 150 may generate a bitstream by performingprobability distribution-based entropy encoding based on values,calculated by the quantization unit 140, and/or coding parameter values,calculated in the encoding procedure. The entropy encoding unit 150 mayoutput the generated bitstream.

The entropy encoding unit 150 may perform entropy encoding oninformation about the pixels of the image and information required todecode the image. For example, the information required to decode theimage may include syntax elements or the like.

When entropy encoding is applied, fewer bits may be assigned to morefrequently occurring symbols, and more bits may be assigned to rarelyoccurring symbols. As symbols are represented by means of thisassignment, the size of a bit string for target symbols to be encodedmay be reduced. Therefore, the compression performance of video encodingmay be improved through entropy encoding.

Further, for entropy encoding, the entropy encoding unit 150 may use acoding method such as exponential Golomb, Context-Adaptive VariableLength Coding (CAVLC), or Context-Adaptive Binary Arithmetic Coding(CABAC). For example, the entropy encoding unit 150 may perform entropyencoding using a Variable Length Coding/Code (VLC) table. For example,the entropy encoding unit 150 may derive a binarization method for atarget symbol. Further, the entropy encoding unit 150 may derive aprobability model for a target symbol/bin. The entropy encoding unit 150may perform arithmetic coding using the derived binarization method, aprobability model, and a context model.

The entropy encoding unit 150 may transform the coefficient of the formof a 2D block into the form of a 1D vector through a transformcoefficient scanning method so as to encode a quantized transformcoefficient level.

The coding parameters may be information required for encoding and/ordecoding. The coding parameters may include information encoded by theencoding apparatus 100 and transferred from the encoding apparatus 100to a decoding apparatus, and may also include information that may bederived in the encoding or decoding procedure. For example, informationtransferred to the decoding apparatus may include syntax elements.

The coding parameters may include not only information (or a flag or anindex), such as a syntax element, which is encoded by the encodingapparatus and is signaled by the encoding apparatus to the decodingapparatus, but also information derived in an encoding or decodingprocess. Further, the coding parameters may include information requiredso as to encode or decode images. For example, the coding parameters mayinclude at least one value, combinations or statistics of a size of aunit/block, a shape/form of a unit/block, a depth of a unit/block,partition information of a unit/block, a partition structure of aunit/block, information indicating whether a unit/block is partitionedin a quad-tree structure, information indicating whether a unit/block ispartitioned in a binary tree structure, a partitioning direction of abinary tree structure (horizontal direction or vertical direction), apartitioning form of a binary tree structure (symmetrical partitioningor asymmetrical partitioning), information indicating whether aunit/block is partitioned in a ternary tree structure, a partitioningdirection of a ternary tree structure (horizontal direction or verticaldirection), a partitioning form of a ternary tree structure (symmetricalpartitioning or asymmetrical partitioning, etc.), information indicatingwhether a unit/block is partitioned in a multi-type tree structure, acombination and a direction (horizontal direction or vertical direction,etc.) of a partitioning of the multi-type tree structure, a partitioningform of a multi-type tree structure (symmetrical partitioning orasymmetrical partitioning, etc.), a partitioning tree (a binary tree ora ternary tree) of the multi-type tree form, a type of a prediction(intra prediction or inter prediction), an intra-predictionmode/direction, an intra luma prediction mode/direction, an intra chromaprediction mode/direction, an intra partitioning information, an interpartitioning information, a coding block partitioning flag, a predictionblock partitioning flag, a transform block partitioning flag, areference sample filtering method, a reference sample filter tap, areference sample filter coefficient, a prediction block filteringmethod, a prediction block filter tap, a prediction block filtercoefficient, a prediction block boundary filtering method, a predictionblock boundary filter tap, a prediction block boundary filtercoefficient, an inter-prediction mode, motion information, a motionvector, a motion vector difference, a reference picture index, aninter-prediction direction, an inter-prediction indicator, a predictionlist utilization flag, a reference picture list, a reference image, aPOC, a motion vector predictor, a motion vector prediction index, amotion vector prediction candidate, a motion vector candidate list,information indicating whether a merge mode is used, a merge index, amerge candidate, a merge candidate list, information indicating whethera skip mode is used, a type of an interpolation filter, a tap of aninterpolation filter, a filter coefficient of an interpolation filter, amagnitude of a motion vector, accuracy of motion vector representation,a transform type, a transform size, information indicating whether afirst transform is used, information indicating whether an additional(secondary) transform is used, first transform selection information (ora first transform index), secondary transform selection information (ora secondary transform index), information indicating a presence orabsence of a residual signal, a coded block pattern, a coded block flag,a quantization parameter, a residual quantization parameter, aquantization matrix, information about an intra-loop filter, informationindicating whether an intra-loop filter is applied, a coefficient of anintra-loop filter, a tap of an intra-loop filter, a shape/form of anintra-loop filter, information indicating whether a deblocking filter isapplied, a coefficient of a deblocking filter, a tap of a deblockingfilter, deblocking filter strength, a shape/form of a deblocking filter,information indicating whether an adaptive sample offset is applied, avalue of an adaptive sample offset, a category of an adaptive sampleoffset, a type of an adaptive sample offset, information indicatingwhether an adaptive in-loop filter is applied, a coefficient of anadaptive in-loop filter, a tap of an adaptive in-loop filter, ashape/form of an adaptive in-loop filter, a binarization/inversebinarization method, a context model, a context model decision method, acontext model update method, information indicating whether a regularmode is performed, information whether a bypass mode is performed, asignificant coefficient flag, a last significant coefficient flag, acoding flag for a coefficient group, a position of a last significantcoefficient, information indicating whether a value of a coefficient isgreater than 1, information indicating whether a value of a coefficientis greater than 2, information indicating whether a value of acoefficient is greater than 3, a remaining coefficient valueinformation, a sign information, a reconstructed luma sample, areconstructed chroma sample, a context bin, a bypass bin, a residualluma sample, a residual chroma sample, a transform coefficient, a lumatransform coefficient, a chroma transform coefficient, a quantizedlevel, a luma quantized level, a chroma quantized level, a transformcoefficient level, a transform coefficient level scanning method, a sizeof a motion vector search region on a side of a decoding apparatus, ashape/form of a motion vector search region on a side of a decodingapparatus, the number of a motion vector search on a side of a decodingapparatus, a size of a CTU, a minimum block size, a maximum block size,a maximum block depth, a minimum block depth, an image display/outputorder, slice identification information, a slice type, slice partitioninformation, tile group identification information, a tile group type, atile group partitioning information, tile identification information, atile type, tile partitioning information, a picture type, bit depth,input sample bit depth, reconstructed sample bit depth, residual samplebit depth, transform coefficient bit depth, quantized level bit depth,information about a luma signal, information about a chroma signal, acolor space of a target block and a color space of a residual block.Further, the above-described coding parameter-related information mayalso be included in the coding parameter. Information used to calculateand/or derive the above-described coding parameter may also be includedin the coding parameter. Information calculated or derived using theabove-described coding parameter may also be included in the codingparameter.

The first transform selection information may indicate a first transformwhich is applied to a target block.

The second transform selection information may indicate a secondtransform which is applied to a target block.

The residual signal may denote the difference between the originalsignal and a prediction signal. Alternatively, the residual signal maybe a signal generated by transforming the difference between theoriginal signal and the prediction signal. Alternatively, the residualsignal may be a signal generated by transforming and quantizing thedifference between the original signal and the prediction signal. Aresidual block may be the residual signal for a block.

Here, signaling information may mean that the encoding apparatus 100includes an entropy-encoded information, generated by performing entropyencoding a flag or an index, in a bitstream, and that the decodingapparatus 200 acquires information by performing entropy decoding on theentropy-encoded information, extracted from the bitstream. Here, theinformation may comprises a flag, an index, etc.

A signal may mean information to be signaled. Hereinafter, informationfor an image and a block may be referred to as a signal. Further,hereinafter, the terms “information” and “signal” may be used to havethe same meaning and may be used interchangeably with each other. Forexample, a specific signal may be a signal representing a specificblock. An original signal may be a signal representing a target block. Aprediction signal may be a signal representing a prediction block. Aresidual signal may be a signal representing a residual block.

A bitstream may include information based on a specific syntax. Theencoding apparatus 100 may generate a bitstream including informationdepending on a specific syntax. The encoding apparatus 200 may acquireinformation from the bitstream depending on a specific syntax.

Since the encoding apparatus 100 performs encoding via inter prediction,the encoded target image may be used as a reference image for additionalimage(s) to be subsequently processed. Therefore, the encoding apparatus100 may reconstruct or decode the encoded target image and store thereconstructed or decoded image as a reference image in the referencepicture buffer 190. For decoding, dequantization and inverse transformon the encoded target image may be processed.

The quantized level may be inversely quantized by the dequantizationunit 160, and may be inversely transformed by the inverse transform unit170. The dequantization unit 160 may generate an inversely quantizedcoefficient by performing inverse transform for the quantized level. Theinverse transform unit 170 may generate a inversely quantized andinversely transformed coefficient by performing inverse transform forthe inversely quantized coefficient.

The inversely quantized and inversely transformed coefficient may beadded to the prediction block by the adder 175. The inversely quantizedand inversely transformed coefficient and the prediction block areadded, and then a reconstructed block may be generated. Here, theinversely quantized and/or inversely transformed coefficient may denotea coefficient on which one or more of dequantization and inversetransform are performed, and may also denote a reconstructed residualblock. Here, the reconstructed block may mean a recovered block or adecoded block.

The reconstructed block may be subjected to filtering through the filterunit 180. The filter unit 180 may apply one or more of a deblockingfilter, a Sample Adaptive Offset (SAO) filter, an Adaptive Loop Filter(ALF), and a Non Local Filter (NLF) to a reconstructed sample, thereconstructed block or a reconstructed picture. The filter unit 180 mayalso be referred to as an “in-loop filter”.

The deblocking filter may eliminate block distortion occurring at theboundaries between blocks in a reconstructed picture. In order todetermine whether to apply the deblocking filter, the number of columnsor rows which are included in a block and which include pixel(s) basedon which it is determined whether to apply the deblocking filter to atarget block may be decided on.

When the deblocking filter is applied to the target block, the appliedfilter may differ depending on the strength of the required deblockingfiltering. In other words, among different filters, a filter decided onin consideration of the strength of deblocking filtering may be appliedto the target block. When a deblocking filter is applied to a targetblock, one or more filters of a long-tap filter, a strong filter, a weakfilter and Gaussian filter may be applied to the target block dependingon the strength of required deblocking filtering.

Also, when vertical filtering and horizontal filtering are performed onthe target block, the horizontal filtering and the vertical filteringmay be processed in parallel.

The SAO may add a suitable offset to the values of pixels to compensatefor coding error. The SAO may perform, for the image to which deblockingis applied, correction that uses an offset in the difference between anoriginal image and the image to which deblocking is applied, on a pixelbasis. To perform an offset correction for an image, a method fordividing the pixels included in the image into a certain number ofregions, determining a region to which an offset is to be applied, amongthe divided regions, and applying an offset to the determined region maybe used, and a method for applying an offset in consideration of edgeinformation of each pixel may also be used.

The ALF may perform filtering based on a value obtained by comparing areconstructed image with an original image. After pixels included in animage have been divided into a predetermined number of groups, filtersto be applied to each group may be determined, and filtering may bedifferentially performed for respective groups. information related towhether to apply an adaptive loop filter may be signaled for each CU.Such information may be signaled for a luma signal. The shapes andfilter coefficients of ALFs to be applied to respective blocks maydiffer for respective blocks. Alternatively, regardless of the featuresof a block, an ALF having a fixed form may be applied to the block.

A non-local filter may perform filtering based on reconstructed blocks,similar to a target block. A region similar to the target block may beselected from a reconstructed picture, and filtering of the target blockmay be performed using the statistical properties of the selectedsimilar region. Information about whether to apply a non-local filtermay be signaled for a Coding Unit (CU). Also, the shapes and filtercoefficients of the non-local filter to be applied to blocks may differdepending on the blocks.

The reconstructed block or the reconstructed image subjected tofiltering through the filter unit 180 may be stored in the referencepicture buffer 190 as a reference picture. The reconstructed blocksubjected to filtering through the filter unit 180 may be a part of areference picture. In other words, the reference picture may be areconstructed picture composed of reconstructed blocks subjected tofiltering through the filter unit 180. The stored reference picture maybe subsequently used for inter prediction or a motion compensation.

FIG. 2 is a block diagram illustrating the configuration of anembodiment of a decoding apparatus to which the present disclosure isapplied.

A decoding apparatus 200 may be a decoder, a video decoding apparatus oran image decoding apparatus.

Referring to FIG. 2 , the decoding apparatus 200 may include an entropydecoding unit 210, a dequantization (inverse quantization) unit 220, aninverse transform unit 230, an intra-prediction unit 240, aninter-prediction unit 250, a switch 245 an adder 255, a filter unit 260,and a reference picture buffer 270.

The decoding apparatus 200 may receive a bitstream output from theencoding apparatus 100. The decoding apparatus 200 may receive abitstream stored in a computer-readable storage medium, and may receivea bitstream that is streamed through a wired/wireless transmissionmedium.

The decoding apparatus 200 may perform decoding on the bitstream in anintra mode and/or an inter mode. Further, the decoding apparatus 200 maygenerate a reconstructed image or a decoded image via decoding, and mayoutput the reconstructed image or decoded image.

For example, switching to an intra mode or an inter mode based on theprediction mode used for decoding may be performed by the switch 245.When the prediction mode used for decoding is an intra mode, the switch245 may be operated to switch to the intra mode. When the predictionmode used for decoding is an inter mode, the switch 245 may be operatedto switch to the inter mode.

The decoding apparatus 200 may acquire a reconstructed residual block bydecoding the input bitstream, and may generate a prediction block. Whenthe reconstructed residual block and the prediction block are acquired,the decoding apparatus 200 may generate a reconstructed block, which isthe target to be decoded, by adding the reconstructed residual block andthe prediction block.

The entropy decoding unit 210 may generate symbols by performing entropydecoding on the bitstream based on the probability distribution of abitstream. The generated symbols may include symbols in a form of aquantized transform coefficient level (i.e., a quantized level or aquantized coefficient). Here, the entropy decoding method may be similarto the above-described entropy encoding method. That is, the entropydecoding method may be the reverse procedure of the above-describedentropy encoding method.

The entropy decoding unit 210 may change a coefficient having aone-dimensional (1D) vector form to a 2D block shape through a transformcoefficient scanning method in order to decode a quantized transformcoefficient level.

For example, the coefficients of the block may be changed to 2D blockshapes by scanning the block coefficients using up-right diagonalscanning. Alternatively, which one of up-right diagonal scanning,vertical scanning, and horizontal scanning is to be used may bedetermined depending on the size and/or the intra-prediction mode of thecorresponding block.

The quantized coefficient may be inversely quantized by thedequantization unit 220. The dequantization unit 220 may generate aninversely quantized coefficient by performing dequantization on thequantized coefficient. Further, the inversely quantized coefficient maybe inversely transformed by the inverse transform unit 230. The inversetransform unit 230 may generate a reconstructed residual block byperforming an inverse transform on the inversely quantized coefficient.As a result of performing dequantization and the inverse transform onthe quantized coefficient, the reconstructed residual block may begenerated. Here, the dequantization unit 220 may apply a quantizationmatrix to the quantized coefficient when generating the reconstructedresidual block.

When the intra mode is used, the intra-prediction unit 240 may generatea prediction block by performing spatial prediction that uses the pixelvalues of previously decoded neighbor blocks adjacent to a target blockfor the target block.

The inter-prediction unit 250 may include a motion compensation unit.Alternatively, the inter-prediction unit 250 may be designated as a“motion compensation unit”.

When the inter mode is used, the motion compensation unit may generate aprediction block by performing motion compensation that uses a motionvector and a reference image stored in the reference picture buffer 270for the target block.

The motion compensation unit may apply an interpolation filter to apartial area of the reference image when the motion vector has a valueother than an integer, and may generate a prediction block using thereference image to which the interpolation filter is applied. In orderto perform motion compensation, the motion compensation unit maydetermine which one of a skip mode, a merge mode, an Advanced MotionVector Prediction (AMVP) mode, and a current picture reference modecorresponds to the motion compensation method used for a PU included ina CU, based on the CU, and may perform motion compensation depending onthe determined mode.

The reconstructed residual block and the prediction block may be addedto each other by the adder 255. The adder 255 may generate areconstructed block by adding the reconstructed residual block to theprediction block.

The reconstructed block may be subjected to filtering through the filterunit 260. The filter unit 260 may apply at least one of a deblockingfilter, an SAO filter, an ALF, and a NLF to the reconstructed block orthe reconstructed image. The reconstructed image may be a pictureincluding the reconstructed block.

The filter unit may output the reconstructed image.

The reconstructed image and/or the reconstructed block subjected tofiltering through the filter unit 260 may be stored as a referencepicture in the reference picture buffer 270. The reconstructed blocksubjected to filtering through the filter unit 260 may be a part of thereference picture. In other words, the reference picture may be an imagecomposed of reconstructed blocks subjected to filtering through thefilter unit 260. The stored reference picture may be subsequently usedfor inter prediction or a motion compensation.

FIG. 3 is a diagram schematically illustrating the partition structureof an image when the image is encoded and decoded.

FIG. 3 may schematically illustrate an example in which a single unit ispartitioned into multiple sub-units.

In order to efficiently partition the image, a Coding Unit (CU) may beused in encoding and decoding. The term “unit” may be used tocollectively designate 1) a block including image samples and 2) asyntax element. For example, the “partitioning of a unit” may mean the“partitioning of a block corresponding to a unit”.

A CU may be used as a base unit for image encoding/decoding. A CU may beused as a unit to which one mode selected from an intra mode and aninter mode in image encoding/decoding is applied. In other words, inimage encoding/decoding, which one of an intra mode and an inter mode isto be applied to each CU may be determined.

Further, a CU may be a base unit in prediction, transform, quantization,inverse transform, dequantization, and encoding/decoding of transformcoefficients.

Referring to FIG. 3 , an image 200 may be sequentially partitioned intounits corresponding to a Largest Coding Unit (LCU), and a partitionstructure may be determined for each LCU. Here, the LCU may be used tohave the same meaning as a Coding Tree Unit (CTU).

The partitioning of a unit may mean the partitioning of a blockcorresponding to the unit. Block partition information may include depthinformation about the depth of a unit. The depth information mayindicate the number of times the unit is partitioned and/or the degreeto which the unit is partitioned. A single unit may be hierarchicallypartitioned into a plurality of sub-units while having depth informationbased on a tree structure. Each of partitioned sub-units may have depthinformation. The depth information may be information indicating thesize of a CU. The depth information may be stored for each CU.

Each CU may have depth information. When the CU is partitioned, CUsresulting from partitioning may have a depth increased from the depth ofthe partitioned CU by 1.

The partition structure may mean the distribution of Coding Units (CUs)to efficiently encode the image in an LCU 310. Such a distribution maybe determined depending on whether a single CU is to be partitioned intomultiple CUs. The number of CUs generated by partitioning may be apositive integer of 2 or more, including 2, 3, 4, 8, 16, etc.

The horizontal size and the vertical size of each of CUs generated bythe partitioning may be less than the horizontal size and the verticalsize of a CU before being partitioned, depending on the number of CUsgenerated by partitioning. For example, the horizontal size and thevertical size of each of CUs generated by the partitioning may be halfof the horizontal size and the vertical size of a CU before beingpartitioned.

Each partitioned CU may be recursively partitioned into four CUs in thesame way. Via the recursive partitioning, at least one of the horizontalsize and the vertical size of each partitioned CU may be reducedcompared to at least one of the horizontal size and the vertical size ofthe CU before being partitioned.

The partitioning of a CU may be recursively performed up to a predefineddepth or a predefined size.

For example, the depth of a CU may have a value ranging from 0 to 3. Thesize of the CU may range from a size of 64×64 to a size of 8×8 dependingon the depth of the CU.

For example, the depth of an LCU 310 may be 0, and the depth of aSmallest Coding Unit (SCU) may be a predefined maximum depth. Here, asdescribed above, the LCU may be the CU having the maximum coding unitsize, and the SCU may be the CU having the minimum coding unit size.

Partitioning may start at the LCU 310, and the depth of a CU may beincreased by 1 whenever the horizontal and/or vertical sizes of the CUare reduced by partitioning.

For example, for respective depths, a CU that is not partitioned mayhave a size of 2N×2N. Further, in the case of a CU that is partitioned,a CU having a size of 2N×2N may be partitioned into four CUs, eachhaving a size of N×N. The value of N may be halved whenever the depth isincreased by 1.

Referring to FIG. 3 , an LCU having a depth of 0 may have 64×64 pixelsor 64×64 blocks. 0 may be a minimum depth. An SCU having a depth of 3may have 8×8 pixels or 8×8 blocks. 3 may be a maximum depth. Here, a CUhaving 64×64 blocks, which is the LCU, may be represented by a depth of0. A CU having 32×32 blocks may be represented by a depth of 1. A CUhaving 16×16 blocks may be represented by a depth of 2. A CU having 8×8blocks, which is the SCU, may be represented by a depth of 3.

Information about whether the corresponding CU is partitioned may berepresented by the partition information of the CU. The partitioninformation may be 1-bit information. All CUs except the SCU may includepartition information. For example, the value of the partitioninformation of a CU that is not partitioned may be a first value. Thevalue of the partition information of a CU that is partitioned may be asecond value. When the partition information indicates whether a CU ispartitioned or not, the first value may be “0” and the second value maybe “1”.

For example, when a single CU is partitioned into four CUs, thehorizontal size and vertical size of each of four CUs generated bypartitioning may be half the horizontal size and the vertical size ofthe CU before being partitioned. When a CU having a 32×32 size ispartitioned into four CUs, the size of each of four partitioned CUs maybe 16×16. When a single CU is partitioned into four CUs, it may beconsidered that the CU has been partitioned in a quad-tree structure. Inother words, it may be considered that a quad-tree partition has beenapplied to a CU.

For example, when a single CU is partitioned into two CUs, thehorizontal size or the vertical size of each of two CUs generated bypartitioning may be half the horizontal size or the vertical size of theCU before being partitioned. When a CU having a 32×32 size is verticallypartitioned into two CUs, the size of each of two partitioned CUs may be16×32. When a CU having a 32×32 size is horizontally partitioned intotwo CUs, the size of each of two partitioned CUs may be 32×16. When asingle CU is partitioned into two CUs, it may be considered that the CUhas been partitioned in a binary-tree structure. In other words, it maybe considered that a binary-tree partition has been applied to a CU.

For example, when a single CU is partitioned (or split) into three CUs,the original CU before being partitioned is partitioned so that thehorizontal size or vertical size thereof is divided at a ratio of 1:2:1,thus enabling three sub-CUs to be generated. For example, when a CUhaving a 16×32 size is horizontally partitioned into three sub-CUs, thethree sub-CUs resulting from the partitioning may have sizes of 16×8,16×16, and 16×8, respectively, in a direction from the top to thebottom. For example, when a CU having a 32×32 size is verticallypartitioned into three sub-CUs, the three sub-CUs resulting from thepartitioning may have sizes of 8×32, 16×32, and 8×32, respectively, in adirection from the left to the right. When a single CU is partitionedinto three CUs, it may be considered that the CU is partitioned in aternary-tree form. In other words, it may be considered that aternary-tree partition has been applied to the CU.

Both of quad-tree partitioning and binary-tree partitioning are appliedto the LCU 310 of FIG. 3 .

In the encoding apparatus 100, a Coding Tree Unit (CTU) having a size of64×64 may be partitioned into multiple smaller CUs by a recursivequad-tree structure. A single CU may be partitioned into four CUs havingthe same size. Each CU may be recursively partitioned, and may have aquad-tree structure.

By the recursive partitioning of a CU, an optimal partitioning methodthat incurs a minimum rate-distortion cost may be selected.

The Coding Tree Unit (CTU) 320 in FIG. 3 is an example of a CTU to whichall of a quad-tree partition, a binary-tree partition, and aternary-tree partition are applied.

As described above, in order to partition a CTU, at least one of aquad-tree partition, a binary-tree partition, and a ternary-treepartition may be applied to the CTU. Partitions may be applied based onspecific priority.

For example, a quad-tree partition may be preferentially applied to theCTU. A CU that cannot be partitioned in a quad-tree form any further maycorrespond to a leaf node of a quad-tree. A CU corresponding to the leafnode of the quad-tree may be a root node of a binary tree and/or aternary tree. That is, the CU corresponding to the leaf node of thequad-tree may be partitioned in a binary-tree form or a ternary-treeform, or may not be partitioned any further. In this case, each CU,which is generated by applying a binary-tree partition or a ternary-treepartition to the CU corresponding to the leaf node of a quad-tree, isprevented from being subjected again to quad-tree partitioning, thuseffectively performing partitioning of a block and/or signaling of blockpartition information.

The partition of a CU corresponding to each node of a quad-tree may besignaled using quad-partition information. Quad-partition informationhaving a first value (e.g., “1”) may indicate that the corresponding CUis partitioned in a quad-tree form. Quad-partition information having asecond value (e.g., “0”) may indicate that the corresponding CU is notpartitioned in a quad-tree form. The quad-partition information may be aflag having a specific length (e.g., 1 bit).

Priority may not exist between a binary-tree partition and aternary-tree partition. That is, a CU corresponding to the leaf node ofa quad-tree may be partitioned in a binary-tree form or a ternary-treeform. Also, the CU generated through a binary-tree partition or aternary-tree partition may be further partitioned in a binary-tree formor a ternary-tree form, or may not be partitioned any further.

Partitioning performed when priority does not exist between abinary-tree partition and a ternary-tree partition may be referred to asa “multi-type tree partition”. That is, a CU corresponding to the leafnode of a quad-tree may be the root node of a multi-type tree.Partitioning of a CU corresponding to each node of the multi-type treemay be signaled using at least one of information indicating whether theCU is partitioned in a multi-type tree, partition direction information,and partition tree information. For partitioning of a CU correspondingto each node of a multi-type tree, information indicating whetherpartitioning in the multi-type tree is performed, partition directioninformation, and partition tree information may be sequentiallysignaled.

For example, information indicating whether a CU is partitioned in amulti-type tree and having a first value (e.g., “1”) may indicate thatthe corresponding CU is partitioned in a multi-type tree form.Information indicating whether a CU is partitioned in a multi-type treeand having a second value (e.g., “0”) may indicate that thecorresponding CU is not partitioned in a multi-type tree form.

When a CU corresponding to each node of a multi-type tree is partitionedin a multi-type tree form, the corresponding CU may further includepartition direction information.

The partition direction information may indicate the partition directionof the multi-type tree partition. Partition direction information havinga first value (e.g., “1”) may indicate that the corresponding CU ispartitioned in a vertical direction. Partition direction informationhaving a second value (e.g., “0”) may indicate that the corresponding CUis partitioned in a horizontal direction.

When a CU corresponding to each node of a multi-type tree is partitionedin a multi-type tree form, the corresponding CU may further includepartition-tree information. The partition-tree information may indicatethe tree that is used for a multi-type tree partition.

For example, partition-tree information having a first value (e.g., “1”)may indicate that the corresponding CU is partitioned in a binary-treeform. Partition-tree information having a second value (e.g., “0”) mayindicate that the corresponding CU is partitioned in a ternary-treeform.

Here, each of the above-described information indicating whetherpartitioning in the multi-type tree is performed, partition-treeinformation, and partition direction information may be a flag having aspecific length (e.g., 1 bit).

At least one of the above-described quad-partition information,information indicating whether partitioning in the multi-type tree isperformed, partition direction information, and partition-treeinformation may be entropy-encoded and/or entropy-decoded. In order toperform entropy encoding/decoding of such information, information of aneighbor CU adjacent to a target CU may be used.

For example, it may be considered that there is a high probability thatthe partition form of a left CU and/or an above CU (i.e.,partitioning/non-partitioning, a partition tree and/or a partitiondirection) and the partition form of a target CU will be similar to eachother. Therefore, based on the information of a neighbor CU, contextinformation for entropy encoding and/or entropy decoding of theinformation of the target CU may be derived. Here, the information ofthe neighbor CU may include at least one of 1) quad-partitioninformation of the neighbor CU, 2) information indicating whether theneighbor CU is partitioned in a multi-type tree, 3) partition directioninformation of the neighbor CU, and 4) partition-tree information of theneighbor CU.

In another embodiment, of a binary-tree partition and a ternary-treepartition, the binary-tree partition may be preferentially performed.That is, the binary-tree partition may be first applied, and then a CUcorresponding to the leaf node of a binary tree may be set to the rootnode of a ternary tree. In this case, a quad-tree partition or abinary-tree partition may not be performed on the CU corresponding tothe node of the ternary tree.

A CU, which is not partitioned any further through a quad-treepartition, a binary-tree partition, and/or a ternary-tree partition, maybe the unit of encoding, prediction and/or transform. That is, the CUmay not be partitioned any further for prediction and/or transform.Therefore, a partition structure for partitioning the CU into PredictionUnits (PUs) and/or Transform Units (TUs), partition information thereof,etc. may not be present in a bitstream.

However, when the size of a CU, which is the unit of partitioning, isgreater than the size of a maximum transform block, the CU may berecursively partitioned until the size of the CU becomes less than orequal to the size of the maximum transform block. For example, when thesize of a CU is 64×64 and the size of the maximum transform block is32×32, the CU may be partitioned into four 32×32 blocks so as to performa transform. For example, when the size of a CU is 32×64 and the size ofthe maximum transform block is 32×32, the CU may be partitioned into two32×32 blocks.

In this case, information indicating whether a CU is partitioned for atransform may not be separately signaled. Without signaling, whether aCU is partitioned may be determined via a comparison between thehorizontal size (and/or vertical size) of the CU and the horizontal size(and/or vertical size) of the maximum transform block. For example, whenthe horizontal size of the CU is greater than the horizontal size of themaximum transform block, the CU may be vertically bisected. Further,when the vertical size of the CU is greater than the vertical size ofthe maximum transform block, the CU may be horizontally bisected.

Information about the maximum size and/or minimum size of a CU andinformation about the maximum size and/or minimum size of a transformblock may be signaled or determined at a level higher than that of theCU. For example, the higher level may be a sequence level, a picturelevel, a tile level, a tile group level or a slice level. For example,the minimum size of the CU may be set to 4×4. For example, the maximumsize of the transform block may be set to 64×64. For example, themaximum size of the transform block may be set to 4×4.

Information about the minimum size of a CU corresponding to the leafnode of a quad-tree (i.e., the minimum size of the quad-tree) and/orinformation about the maximum depth of a path from the root node to theleaf node of a multi-type tree (i.e., the maximum depth of a multi-typetree) may be signaled or determined at a level higher than that of theCU. For example, the higher level may be a sequence level, a picturelevel, a slice level, a tile group level or a tile level. Informationabout the minimum size of a quad-tree and/or information about themaximum depth of a multi-type tree may be separately signaled ordetermined at each of an intra-slice level and an inter-slice level.

Information about the difference between the size of a CTU and themaximum size of a transform block may be signaled or determined at alevel higher than that of a CU. For example, the higher level may be asequence level, a picture level, a slice level, a tile group level or atile level. Information about the maximum size of a CU corresponding toeach node of a binary tree (i.e., the maximum size of the binary tree)may be determined based on the size and the difference information of aCTU. The maximum size of a CU corresponding to each node of a ternarytree (i.e., the maximum size of the ternary tree) may have differentvalues depending on the type of slice. For example, the maximum size ofthe ternary tree at an intra-slice level may be 32×32. For example, themaximum size of the ternary tree at an inter-slice level may be 128×128.For example, the minimum size of a CU corresponding to each node of abinary tree (i.e., the minimum size of the binary tree) and/or theminimum size of a CU corresponding to each node of a ternary tree (i.e.,the minimum size of the ternary tree) may be set to the minimum size ofa CU.

In a further example, the maximum size of a binary tree and/or themaximum size of a ternary tree may be signaled or determined at a slicelevel. Also, the minimum size of a binary tree and/or the minimum sizeof a ternary tree may be signaled or determined at a slice level.

Based on the above-described various block sizes and depths,quad-partition information, information indicating whether partitioningin a multi-type tree is performed, partition tree information and/orpartition direction information may or may not be present in abitstream.

For example, when the size of a CU is not greater than the minimum sizeof a quad-tree, the CU may not include quad-partition information, andquad-partition information of the CU may be inferred as a second value.

For example, when the size of a CU corresponding to each node of amulti-type tree (horizontal size and vertical size) is greater than themaximum size of a binary tree (horizontal size and vertical size) and/orthe maximum size of a ternary tree (horizontal size and vertical size),the CU may not be partitioned in a binary-tree form and/or aternary-tree form. By means of this determination manner, informationindicating whether partitioning in a multi-type tree is performed maynot be signaled, but may be inferred as a second value.

Alternatively, when the size of a CU corresponding to each node of amulti-type tree (horizontal size and vertical size) is equal to theminimum size of a binary tree (horizontal size and vertical size), orwhen the size of a CU (horizontal size and vertical size) is equal totwice the minimum size of a ternary tree (horizontal size and verticalsize), the CU may not be partitioned in a binary tree form and/or aternary tree form. By means of this determination manner, informationindicating whether partitioning in a multi-type tree is performed maynot be signaled, but may be inferred as a second value. The reason forthis is that, when a CU is partitioned in a binary tree form and/or aternary tree form, a CU smaller than the minimum size of the binary treeand/or the minimum size of the ternary tree is generated.

Alternatively, a binary-tree partition or a ternary-tree partition maybe limited based on the size of a virtual pipeline data unit (i.e., thesize of a pipeline buffer). For example, when a CU is partitioned intosub-CUs unsuitable for the size of a pipeline buffer through abinary-tree partition or a ternary-tree partition, a binary-treepartition or a ternary-tree partition may be limited. The size of thepipeline buffer may be equal to the maximum size of a transform block(e.g., 64×64).

For example, when the size of the pipeline buffer is 64×64, thefollowing partitions may be limited.

-   -   Ternary-tree partition for N×M CU (where N and/or M are 128)    -   Horizontal binary-tree partition for 128×N CU (where N<=64)    -   Vertical binary-tree partition for N×128 CU (where N<=64)

Alternatively, when the depth of a CU corresponding to each node of amulti-type tree is equal to the maximum depth of the multi-type tree,the CU may not be partitioned in a binary-tree form and/or aternary-tree form. By means of this determination manner, informationindicating whether partitioning in a multi-type tree is performed maynot be signaled, but may be inferred as a second value.

Alternatively, information indicating whether partitioning in amulti-type tree is performed may be signaled only when at least one of avertical binary-tree partition, a horizontal binary-tree partition, avertical ternary-tree partition, and a horizontal ternary-tree partitionis possible for a CU corresponding to each node of a multi-type tree.Otherwise, the CU may not be partitioned in a binary-tree form and/or aternary-tree form. By means of this determination manner, informationindicating whether partitioning in a multi-type tree is performed maynot be signaled, but may be inferred as a second value.

Alternatively, partition direction information may be signaled only whenboth a vertical binary-tree partition and a horizontal binary-treepartition are possible or only when both a vertical ternary-treepartition and a horizontal ternary-tree partition are possible, for a CUcorresponding to each node of a multi-type tree. Otherwise, thepartition direction information may not be signaled, but may be inferredas a value indicating the direction in which the CU can be partitioned.

Alternatively, partition tree information may be signaled only when botha vertical binary-tree partition and a vertical ternary-tree partitionare possible or only when both a horizontal binary-tree partition and ahorizontal ternary-tree partition are possible, for a CU correspondingto each node of a multi-type tree. Otherwise, the partition treeinformation may not be signaled, but may be inferred as a valueindicating a tree that can be applied to the partition of the CU.

FIG. 4 is a diagram illustrating the form of a Prediction Unit that aCoding Unit can include.

When, among CUs partitioned from an LCU, a CU, which is not partitionedany further, may be divided into one or more Prediction Units (PUs).Such division is also referred to as “partitioning”.

A PU may be a base unit for prediction. A PU may be encoded and decodedin any one of a skip mode, an inter mode, and an intra mode. A PU may bepartitioned into various shapes depending on respective modes. Forexample, the target block, described above with reference to FIG. 1 ,and the target block, described above with reference to FIG. 2 , mayeach be a PU.

A CU may not be split into PUs. When the CU is not split into PUs, thesize of the CU and the size of a PU may be equal to each other.

In a skip mode, partitioning may not be present in a CU. In the skipmode, a 2N×2N mode 410, in which the sizes of a PU and a CU areidentical to each other, may be supported without partitioning.

In an inter mode, 8 types of partition shapes may be present in a CU.For example, in the inter mode, the 2N×2N mode 410, a 2N×N mode 415, anN×2N mode 420, an N×N mode 425, a 2N×nU mode 430, a 2N×nD mode 435, annL×2N mode 440, and an nR×2N mode 445 may be supported.

In an intra mode, the 2N×2N mode 410 and the N×N mode 425 may besupported.

In the 2N×2N mode 410, a PU having a size of 2N×2N may be encoded. ThePU having a size of 2N×2N may mean a PU having a size identical to thatof the CU. For example, the PU having a size of 2N×2N may have a size of64×64, 32×32, 16×16 or 8×8.

In the N×N mode 425, a PU having a size of N×N may be encoded.

For example, in intra prediction, when the size of a PU is 8×8, fourpartitioned PUs may be encoded. The size of each partitioned PU may be4×4.

When a PU is encoded in an intra mode, the PU may be encoded using anyone of multiple intra-prediction modes. For example, HEVC technology mayprovide 35 intra-prediction modes, and the PU may be encoded in any oneof the 35 intra-prediction modes.

Which one of the 2N×2N mode 410 and the N×N mode 425 is to be used toencode the PU may be determined based on rate-distortion cost.

The encoding apparatus 100 may perform an encoding operation on a PUhaving a size of 2N×2N. Here, the encoding operation may be theoperation of encoding the PU in each of multiple intra-prediction modesthat can be used by the encoding apparatus 100. Through the encodingoperation, the optimal intra-prediction mode for a PU having a size of2N×2N may be derived. The optimal intra-prediction mode may be anintra-prediction mode in which a minimum rate-distortion cost occursupon encoding the PU having a size of 2N×2N, among multipleintra-prediction modes that can be used by the encoding apparatus 100.

Further, the encoding apparatus 100 may sequentially perform an encodingoperation on respective PUs obtained from N×N partitioning. Here, theencoding operation may be the operation of encoding a PU in each ofmultiple intra-prediction modes that can be used by the encodingapparatus 100. By means of the encoding operation, the optimalintra-prediction mode for the PU having a size of N×N may be derived.The optimal intra-prediction mode may be an intra-prediction mode inwhich a minimum rate-distortion cost occurs upon encoding the PU havinga size of N×N, among multiple intra-prediction modes that can be used bythe encoding apparatus 100.

The encoding apparatus 100 may determine which of a PU having a size of2N×2N and PUs having sizes of N×N to be encoded based on a comparison ofa rate-distortion cost of the PU having a size of 2N×2N and arate-distortion costs of the PUs having sizes of N×N.

A single CU may be partitioned into one or more PUs, and a PU may bepartitioned into multiple PUs.

For example, when a single PU is partitioned into four PUs, thehorizontal size and vertical size of each of four PUs generated bypartitioning may be half the horizontal size and the vertical size ofthe PU before being partitioned. When a PU having a 32×32 size ispartitioned into four PUs, the size of each of four partitioned PUs maybe 16×16. When a single PU is partitioned into four PUs, it may beconsidered that the PU has been partitioned in a quad-tree structure.

For example, when a single PU is partitioned into two PUs, thehorizontal size or the vertical size of each of two PUs generated bypartitioning may be half the horizontal size or the vertical size of thePU before being partitioned. When a PU having a 32×32 size is verticallypartitioned into two PUs, the size of each of two partitioned PUs may be16×32. When a PU having a 32×32 size is horizontally partitioned intotwo PUs, the size of each of two partitioned PUs may be 32×16. When asingle PU is partitioned into two PUs, it may be considered that the PUhas been partitioned in a binary-tree structure.

FIG. 5 is a diagram illustrating the form of a Transform Unit that canbe included in a Coding Unit.

A Transform Unit (TU) may have a base unit that is used for a procedure,such as transform, quantization, inverse transform, dequantization,entropy encoding, and entropy decoding, in a CU.

A TU may have a square shape or a rectangular shape. A shape of a TU maybe determined based on a size and/or a shape of a CU.

Among CUs partitioned from the LCU, a CU which is not partitioned intoCUs any further may be partitioned into one or more TUs. Here, thepartition structure of a TU may be a quad-tree structure. For example,as shown in FIG. 5 , a single CU 510 may be partitioned one or moretimes depending on the quad-tree structure. By means of thispartitioning, the single CU 510 may be composed of TUs having varioussizes.

It can be considered that when a single CU is split two or more times,the CU is recursively split. Through splitting, a single CU may becomposed of Transform Units (TUs) having various sizes.

Alternatively, a single CU may be split into one or more TUs based onthe number of vertical lines and/or horizontal lines that split the CU.

A CU may be split into symmetric TUs or asymmetric TUs. For splittinginto asymmetric TUs, information about the size and/or shape of each TUmay be signaled from the encoding apparatus 100 to the decodingapparatus 200. Alternatively, the size and/or shape of each TU may bederived from information about the size and/or shape of the CU.

A CU may not be split into TUs. When the CU is not split into TUs, thesize of the CU and the size of a TU may be equal to each other.

A single CU may be partitioned into one or more TUs, and a TU may bepartitioned into multiple TUs.

For example, when a single TU is partitioned into four TUs, thehorizontal size and vertical size of each of four TUs generated bypartitioning may be half the horizontal size and the vertical size ofthe TU before being partitioned. When a TU having a 32×32 size ispartitioned into four TUs, the size of each of four partitioned TUs maybe 16×16. When a single TU is partitioned into four TUs, it may beconsidered that the TU has been partitioned in a quad-tree structure.

For example, when a single TU is partitioned into two TUs, thehorizontal size or the vertical size of each of two TUs generated bypartitioning may be half the horizontal size or the vertical size of theTU before being partitioned. When a TU having a 32×32 size is verticallypartitioned into two TUs, the size of each of two partitioned TUs may be16×32. When a TU having a 32×32 size is horizontally partitioned intotwo TUs, the size of each of two partitioned TUs may be 32×16. When asingle TU is partitioned into two TUs, it may be considered that the TUhas been partitioned in a binary-tree structure.

In a way differing from that illustrated in FIG. 5 , a CU may be split.

For example, a single CU may be split into three CUs. The horizontalsizes or vertical sizes of the three CUs generated from splitting may be¼, ½, and ¼, respectively, of the horizontal size or vertical size ofthe original CU before being split.

For example, when a CU having a 32×32 size is vertically split intothree CUs, the sizes of the three CUs generated from the splitting maybe 8×32, 16×32, and 8×32, respectively. In this way, when a single CU issplit into three CUs, it may be considered that the CU is split in theform of a ternary tree.

One of exemplary splitting forms, that is, quad-tree splitting, binarytree splitting, and ternary tree splitting, may be applied to thesplitting of a CU, and multiple splitting schemes may be combined andused together for splitting of a CU. Here, the case where multiplesplitting schemes are combined and used together may be referred to as“complex tree-format splitting”.

FIG. 6 illustrates the splitting of a block according to an example.

In a video encoding and/or decoding process, a target block may besplit, as illustrated in FIG. 6 . For example, the target block may be aCU.

For splitting of the target block, an indicator indicating splitinformation may be signaled from the encoding apparatus 100 to thedecoding apparatus 200. The split information may be informationindicating how the target block is split.

The split information may be one or more of a split flag (hereinafterreferred to as “split_flag”), a quad-binary flag (hereinafter referredto as “QB_flag”), a quad-tree flag (hereinafter referred to as“quadtree_flag”), a binary tree flag (hereinafter referred to as“binarytree_flag”), and a binary type flag (hereinafter referred to as“Btype_flag”).

“split_flag” may be a flag indicating whether a block is split. Forexample, a split_flag value of 1 may indicate that the correspondingblock is split. A split_flag value of 0 may indicate that thecorresponding block is not split.

“QB_flag” may be a flag indicating which one of a quad-tree form and abinary tree form corresponds to the shape in which the block is split.For example, a QB_flag value of 0 may indicate that the block is splitin a quad-tree form. A QB_flag value of 1 may indicate that the block issplit in a binary tree form. Alternatively, a QB_flag value of 0 mayindicate that the block is split in a binary tree form. A QB_flag valueof 1 may indicate that the block is split in a quad-tree form.

“quadtree_flag” may be a flag indicating whether a block is split in aquad-tree form. For example, a quadtree_flag value of 1 may indicatethat the block is split in a quad-tree form. A quadtree_flag value of 0may indicate that the block is not split in a quad-tree form.

“binarytree_flag” may be a flag indicating whether a block is split in abinary tree form. For example, a binarytree_flag value of 1 may indicatethat the block is split in a binary tree form. A binarytree_flag valueof 0 may indicate that the block is not split in a binary tree form.

“Btype_flag” may be a flag indicating which one of a vertical split anda horizontal split corresponds to a split direction when a block issplit in a binary tree form. For example, a Btype_flag value of 0 mayindicate that the block is split in a horizontal direction. A Btype_flagvalue of 1 may indicate that a block is split in a vertical direction.Alternatively, a Btype_flag value of 0 may indicate that the block issplit in a vertical direction. A Btype_flag value of 1 may indicate thata block is split in a horizontal direction.

For example, the split information of the block in FIG. 6 may be derivedby signaling at least one of quadtree_flag, binarytree_flag, andBtype_flag, as shown in the following Table 1.

TABLE 1 quadtree_flag binarytree_flag Btype_flag 1 0 1 1 0 0 1 0 1 0 0 00 0 0 0 0 0 0 1 0 1 1 0 0 0 0 0

For example, the split information of the block in FIG. 6 may be derivedby signaling at least one of split_flag, QB_flag and Btype_flag, asshown in the following Table 2.

TABLE 2 split_flag QB_flag Btype_flag 1 0 1 1 1 0 0 1 0 1 1 0 0 0 0 0 01 1 0 1 1 0 0 0 0

The splitting method may be limited only to a quad-tree or to a binarytree depending on the size and/or shape of the block. When thislimitation is applied, split_flag may be a flag indicating whether ablock is split in a quad-tree form or a flag indicating whether a blockis split in a binary tree form. The size and shape of a block may bederived depending on the depth information of the block, and the depthinformation may be signaled from the encoding apparatus 100 to thedecoding apparatus 200.

When the size of a block falls within a specific range, only splittingin a quad-tree form may be possible. For example, the specific range maybe defined by at least one of a maximum block size and a minimum blocksize at which only splitting in a quad-tree form is possible.

Information indicating the maximum block size and the minimum block sizeat which only splitting in a quad-tree form is possible may be signaledfrom the encoding apparatus 100 to the decoding apparatus 200 through abitstream. Further, this information may be signaled for at least one ofunits such as a video, a sequence, a picture, a parameter, a tile group,and a slice (or a segment).

Alternatively, the maximum block size and/or the minimum block size maybe fixed sizes predefined by the encoding apparatus 100 and the decodingapparatus 200. For example, when the size of a block is above 64×64 andbelow 256×256, only splitting in a quad-tree form may be possible. Inthis case, split_flag may be a flag indicating whether splitting in aquad-tree form is performed.

When the size of a block is greater than the maximum size of a transformblock, only partitioning in a quad-tree form may be possible. Here, asub-block resulting from partitioning may be at least one of a CU and aTU.

In this case, split_flag may be a flag indicating whether a CU ispartitioned in a quad-tree form.

When the size of a block falls within the specific range, only splittingin a binary tree form or a ternary tree form may be possible. Forexample, the specific range may be defined by at least one of a maximumblock size and a minimum block size at which only splitting in a binarytree form or a ternary tree form is possible.

Information indicating the maximum block size and/or the minimum blocksize at which only splitting in a binary tree form or splitting in aternary tree form is possible may be signaled from the encodingapparatus 100 to the decoding apparatus 200 through a bitstream.Further, this information may be signaled for at least one of units suchas a sequence, a picture, and a slice (or a segment).

Alternatively, the maximum block size and/or the minimum block size maybe fixed sizes predefined by the encoding apparatus 100 and the decodingapparatus 200. For example, when the size of a block is above 8×8 andbelow 16×16, only splitting in a binary tree form may be possible. Inthis case, split_flag may be a flag indicating whether splitting in abinary tree form or a ternary tree form is performed.

The above description of partitioning in a quad-tree form may be equallyapplied to a binary-tree form and/or a ternary-tree form.

The partition of a block may be limited by a previous partition. Forexample, when a block is partitioned in a specific binary-tree form andthen multiple sub-blocks are generated from the partitioning, eachsub-block may be additionally partitioned only in a specific tree form.Here, the specific tree form may be at least one of a binary-tree form,a ternary-tree form, and a quad-tree form.

When the horizontal size or vertical size of a partition block is a sizethat cannot be split further, the above-described indicator may not besignaled.

FIG. 7 is a diagram for explaining an embodiment of an intra-predictionprocess.

Arrows radially extending from the center of the graph in FIG. 7indicate the prediction directions of intra-prediction modes. Further,numbers appearing near the arrows indicate examples of mode valuesassigned to intra-prediction modes or to the prediction directions ofthe intra-prediction modes.

In FIG. 7 , A number 0 may represent a Planar mode which is anon-directional intra prediction mode. A number 1 may represent a DCmode which is a non-directional intra prediction mode

Intra encoding and/or decoding may be performed using a reference sampleof neighbor unit of a target block. The neighbor block may be areconstructed neighbor block. The reference sample may mean a neighborsample.

For example, intra encoding and/or decoding may be performed using thevalue of a reference sample which are included in are reconstructedneighbor block or the coding parameters of the reconstructed neighborblock.

The encoding apparatus 100 and/or the decoding apparatus 200 maygenerate a prediction block by performing intra prediction on a targetblock based on information about samples in a target image. When intraprediction is performed, the encoding apparatus 100 and/or the decodingapparatus 200 may generate a prediction block for the target block byperforming intra prediction based on information about samples in thetarget image. When intra prediction is performed, the encoding apparatus100 and/or the decoding apparatus 200 may perform directional predictionand/or non-directional prediction based on at least one reconstructedreference sample.

A prediction block may be a block generated as a result of performingintra prediction. A prediction block may correspond to at least one of aCU, a PU, and a TU.

The unit of a prediction block may have a size corresponding to at leastone of a CU, a PU, and a TU. The prediction block may have a squareshape having a size of 2N×2N or N×N. The size of N×N may include sizesof 4×4, 8×8, 16×16, 32×32, 64×64, or the like.

Alternatively, a prediction block may a square block having a size of2×2, 4×4, 8×8, 16×16, 32×32, 64×64 or the like or a rectangular blockhaving a size of 2×8, 4×8, 2×16, 4×16, 8×16, or the like.

Intra prediction may be performed in consideration of theintra-prediction mode for the target block. The number ofintra-prediction modes that the target block can have may be apredefined fixed value, and may be a value determined differentlydepending on the attributes of a prediction block. For example, theattributes of the prediction block may include the size of theprediction block, the type of prediction block, etc. Further, theattribute of a prediction block may indicate a coding parameter for theprediction block.

For example, the number of intra-prediction modes may be fixed at Nregardless of the size of a prediction block. Alternatively, the numberof intra-prediction modes may be, for example, 3, 5, 9, 17, 34, 35, 36,65, 67 or 95.

The intra-prediction modes may be non-directional modes or directionalmodes.

For example, the intra-prediction modes may include two non-directionalmodes and 65 directional modes corresponding to numbers 0 to 66illustrated in FIG. 7 .

For example, the intra-prediction modes may include two non-directionalmodes and 93 directional modes corresponding to numbers −14 to 80illustrated in FIG. 7 in a case that a specific intra prediction methodis used.

The two non-directional modes may include a DC mode and a planar mode.

A directional mode may be a prediction mode having a specific directionor a specific angle. The directional mode may also be referred to as an“angular mode”.

An intra-prediction mode may be represented by at least one of a modenumber, a mode value, a mode angle, and a mode direction. In otherwords, the terms “(mode) number of the intra-prediction mode”, “(mode)value of the intra-prediction mode”, “(mode) angle of theintra-prediction mode”, and “(mode) direction of the intra-predictionmode” may be used to have the same meaning, and may be usedinterchangeably with each other.

The number of intra-prediction modes may be M. The value of M may be 1or more. In other words, the number of intra-prediction modes may be M,which includes the number of non-directional modes and the number ofdirectional modes.

The number of intra-prediction modes may be fixed to M regardless of thesize and/or the color component of a block. For example, the number ofintra-prediction modes may be fixed at any one of 35 and 67 regardlessof the size of a block.

Alternatively, the number of intra-prediction modes may differ dependingon the shape, the size and/or the type of the color component of ablock.

For example, in FIG. 7 , directional prediction modes illustrated asdashed lines may be applied only for a prediction for a non-squareblock.

For example, the larger the size of the block, the greater the number ofintra-prediction modes. Alternatively, the larger the size of the block,the smaller the number of intra-prediction modes. When the size of theblock is 4×4 or 8×8, the number of intra-prediction modes may be 67.When the size of the block is 16×16, the number of intra-predictionmodes may be 35. When the size of the block is 32×32, the number ofintra-prediction modes may be 19. When the size of a block is 64×64, thenumber of intra-prediction modes may be 7.

For example, the number of intra prediction modes may differ dependingon whether a color component is a luma signal or a chroma signal.Alternatively, the number of intra-prediction modes corresponding to aluma component block may be greater than the number of intra-predictionmodes corresponding to a chroma component block.

For example, in a vertical mode having a mode value of 50, predictionmay be performed in a vertical direction based on the pixel value of areference sample. For example, in a horizontal mode having a mode valueof 18, prediction may be performed in a horizontal direction based onthe pixel value of a reference sample.

Even in directional modes other than the above-described mode, theencoding apparatus 100 and the decoding apparatus 200 may perform intraprediction on a target unit using reference samples depending on anglescorresponding to the directional modes.

Intra-prediction modes located on a right side with respect to thevertical mode may be referred to as ‘vertical-right modes’.Intra-prediction modes located below the horizontal mode may be referredto as ‘horizontal-below modes’. For example, in FIG. 7 , theintra-prediction modes in which a mode value is one of 51, 52, 53, 54,55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, and 66 may be vertical-rightmodes. Intra-prediction modes in which a mode value is one of 2, 3, 4,5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, and 17 may behorizontal-below modes.

The non-directional mode may include a DC mode and a planar mode. Forexample, a value of the DC mode may be 1. A value of the planar mode maybe 0.

The directional mode may include an angular mode. Among the plurality ofthe intra prediction modes, remaining modes except for the DC mode andthe planar mode may be directional modes.

When the intra-prediction mode is a DC mode, a prediction block may begenerated based on the average of pixel values of a plurality ofreference pixels. For example, a value of a pixel of a prediction blockmay be determined based on the average of pixel values of a plurality ofreference pixels.

The number of above-described intra-prediction modes and the mode valuesof respective intra-prediction modes are merely exemplary. The number ofabove-described intra-prediction modes and the mode values of respectiveintra-prediction modes may be defined differently depending on theembodiments, implementation and/or requirements.

In order to perform intra prediction on a target block, the step ofchecking whether samples included in a reconstructed neighbor block canbe used as reference samples of a target block may be performed. When asample that cannot be used as a reference sample of the target block ispresent among samples in the neighbor block, a value generated viacopying and/or interpolation that uses at least one sample value, amongthe samples included in the reconstructed neighbor block, may replacethe sample value of the sample that cannot be used as the referencesample. When the value generated via copying and/or interpolationreplaces the sample value of the existing sample, the sample may be usedas the reference sample of the target block.

When intra prediction is used, a filter may be applied to at least oneof a reference sample and a prediction sample based on at least one ofthe intra-prediction mode and the size of the target block.

The type of filter to be applied to at least one of a reference sampleand a prediction sample may differ depending on at least one of theintra-prediction mode of a target block, the size of the target block,and the shape of the target block. The types of filters may beclassified depending on one or more of the length of filter tap, thevalue of a filter coefficient, and filter strength. The length of filtertap may mean the number of filter taps. Also, the number of filter tapmay mean the length of the filter.

When the intra-prediction mode is a planar mode, a sample value of aprediction target block may be generated using a weighted sum of anabove reference sample of the target block, a left reference sample ofthe target block, an above-right reference sample of the target block,and a below-left reference sample of the target block depending on thelocation of the prediction target sample in the prediction block whenthe prediction block of the target block is generated.

When the intra-prediction mode is a DC mode, the average of referencesamples above the target block and the reference samples to the left ofthe target block may be used when the prediction block of the targetblock is generated. Also, filtering using the values of referencesamples may be performed on specific rows or specific columns in thetarget block. The specific rows may be one or more upper rows adjacentto the reference sample. The specific columns may be one or more leftcolumns adjacent to the reference sample.

When the intra-prediction mode is a directional mode, a prediction blockmay be generated using the above reference samples, left referencesamples, above-right reference sample and/or below-left reference sampleof the target block.

In order to generate the above-described prediction sample,real-number-based interpolation may be performed.

The intra-prediction mode of the target block may be predicted fromintra prediction mode of a neighbor block adjacent to the target block,and the information used for prediction may be entropy-encoded/decoded.

For example, when the intra-prediction modes of the target block and theneighbor block are identical to each other, it may be signaled, using apredefined flag, that the intra-prediction modes of the target block andthe neighbor block are identical.

For example, an indicator for indicating an intra-prediction modeidentical to that of the target block, among intra-prediction modes ofmultiple neighbor blocks, may be signaled.

When the intra-prediction modes of the target block and a neighbor blockare different from each other, information about the intra-predictionmode of the target block may be encoded and/or decoded using entropyencoding and/or decoding.

FIG. 8 is a diagram illustrating reference samples used in anintra-prediction procedure.

Reconstructed reference samples used for intra prediction of the targetblock may include below-left reference samples, left reference samples,an above-left corner reference sample, above reference samples, andabove-right reference samples.

For example, the left reference samples may mean reconstructed referencepixels adjacent to the left side of the target block. The abovereference samples may mean reconstructed reference pixels adjacent tothe top of the target block. The above-left corner reference sample maymean a reconstructed reference pixel located at the above-left corner ofthe target block. The below-left reference samples may mean referencesamples located below a left sample line composed of the left referencesamples, among samples located on the same line as the left sample line.The above-right reference samples may mean reference samples located tothe right of an above sample line composed of the above referencesamples, among samples located on the same line as the above sampleline.

When the size of a target block is N×N, the numbers of the below-leftreference samples, the left reference samples, the above referencesamples, and the above-right reference samples may each be N.

By performing intra prediction on the target block, a prediction blockmay be generated. The generation of the prediction block may include thedetermination of the values of pixels in the prediction block. The sizesof the target block and the prediction block may be equal.

The reference samples used for intra prediction of the target block mayvary depending on the intra-prediction mode of the target block. Thedirection of the intra-prediction mode may represent a dependencerelationship between the reference samples and the pixels of theprediction block. For example, the value of a specified reference samplemay be used as the values of one or more specified pixels in theprediction block. In this case, the specified reference sample and theone or more specified pixels in the prediction block may be the sampleand pixels which are positioned in a straight line in the direction ofan intra-prediction mode. In other words, the value of the specifiedreference sample may be copied as the value of a pixel located in adirection reverse to the direction of the intra-prediction mode.Alternatively, the value of a pixel in the prediction block may be thevalue of a reference sample located in the direction of theintra-prediction mode with respect to the location of the pixel.

In an example, when the intra-prediction mode of a target block is avertical mode, the above reference samples may be used for intraprediction. When the intra-prediction mode is the vertical mode, thevalue of a pixel in the prediction block may be the value of a referencesample vertically located above the location of the pixel. Therefore,the above reference samples adjacent to the top of the target block maybe used for intra prediction. Furthermore, the values of pixels in onerow of the prediction block may be identical to those of the abovereference samples.

In an example, when the intra-prediction mode of a target block is ahorizontal mode, the left reference samples may be used for intraprediction. When the intra-prediction mode is the horizontal mode, thevalue of a pixel in the prediction block may be the value of a referencesample horizontally located left to the location of the pixel.Therefore, the left reference samples adjacent to the left of the targetblock may be used for intra prediction. Furthermore, the values ofpixels in one column of the prediction block may be identical to thoseof the left reference samples.

In an example, when the mode value of the intra-prediction mode of thecurrent block is 34, at least some of the left reference samples, theabove-left corner reference sample, and at least some of the abovereference samples may be used for intra prediction. When the mode valueof the intra-prediction mode is 34, the value of a pixel in theprediction block may be the value of a reference sample diagonallylocated at the above-left corner of the pixel.

Further, At least a part of the above-right reference samples may beused for intra prediction in a case that an intra prediction mode ofwhich a mode value is a value ranging from 52 to 66.

Further, At least a part of the below-left reference samples may be usedfor intra prediction in a case that an intra prediction mode of which amode value is a value ranging from 2 to 17.

Further, the above-left corner reference sample may be used for intraprediction in a case that an intra prediction mode of which a mode valueis a value ranging from 19 to 49.

The number of reference samples used to determine the pixel value of onepixel in the prediction block may be either 1, or 2 or more.

As described above, the pixel value of a pixel in the prediction blockmay be determined depending on the location of the pixel and thelocation of a reference sample indicated by the direction of theintra-prediction mode. When the location of the pixel and the locationof the reference sample indicated by the direction of theintra-prediction mode are integer positions, the value of one referencesample indicated by an integer position may be used to determine thepixel value of the pixel in the prediction block.

When the location of the pixel and the location of the reference sampleindicated by the direction of the intra-prediction mode are not integerpositions, an interpolated reference sample based on two referencesamples closest to the location of the reference sample may begenerated. The value of the interpolated reference sample may be used todetermine the pixel value of the pixel in the prediction block. In otherwords, when the location of the pixel in the prediction block and thelocation of the reference sample indicated by the direction of theintra-prediction mode indicate the location between two referencesamples, an interpolated value based on the values of the two samplesmay be generated.

The prediction block generated via prediction may not be identical to anoriginal target block. In other words, there may be a prediction errorwhich is the difference between the target block and the predictionblock, and there may also be a prediction error between the pixel of thetarget block and the pixel of the prediction block.

Hereinafter, the terms “difference”, “error”, and “residual” may be usedto have the same meaning, and may be used interchangeably with eachother.

For example, in the case of directional intra prediction, the longer thedistance between the pixel of the prediction block and the referencesample, the greater the prediction error that may occur. Such aprediction error may result in discontinuity between the generatedprediction block and neighbor blocks.

In order to reduce the prediction error, filtering for the predictionblock may be used. Filtering may be configured to adaptively apply afilter to an area, regarded as having a large prediction error, in theprediction block. For example, the area regarded as having a largeprediction error may be the boundary of the prediction block. Further,an area regarded as having a large prediction error in the predictionblock may differ depending on the intra-prediction mode, and thecharacteristics of filters may also differ depending thereon.

As illustrated in FIG. 8 , for intra prediction of a target block, atleast one of reference line 0 to reference line 3 may be used.

Each reference line in FIG. 8 may indicate a reference sample linecomprising one or more reference samples. As the number of the referenceline is lower, a line of reference samples closer to a target block maybe indicated.

Samples in segment A and segment F may be acquired through padding thatuses samples closest to the target block in segment B and segment Einstead of being acquired from reconstructed neighbor blocks.

Index information indicating a reference sample line to be used forintra-prediction of the target block may be signaled. The indexinformation may indicate a reference sample line to be used forintra-prediction of the target block, among multiple reference samplelines. For example, the index information may have a value correspondingto any one of 0 to 3.

When the top boundary of the target block is the boundary of a CTU, onlyreference sample line 0 may be available. Therefore, in this case, indexinformation may not be signaled. When an additional reference sampleline other than reference sample line 0 is used, filtering of aprediction block, which will be described later, may not be performed.

In the case of inter-color intra prediction, a prediction block for atarget block of a second color component may be generated based on thecorresponding reconstructed block of a first color component.

For example, the first color component may be a luma component, and thesecond color component may be a chroma component.

In order to perform inter-color intra prediction, parameters for alinear model between the first color component and the second colorcomponent may be derived based on a template.

The template may include reference samples above the target block (abovereference samples) and/or reference samples to the left of the targetblock (left reference samples), and may include above reference samplesand/or left reference samples of a reconstructed block of the firstcolor component, which correspond to the reference samples.

For example, parameters for a linear model may be derived using 1) thevalue of the sample of a first color component having the maximum value,among the samples in the template, 2) the value of the sample of asecond color component corresponding to the sample of the first colorcomponent, 3) the value of the sample of a first color component havingthe minimum value, among the samples in the template, and 4) the valueof the sample of a second color component corresponding to the sample ofthe first color component.

When the parameters for the linear model are derived, a prediction blockfor the target block may be generated by applying the correspondingreconstructed block to the linear model.

Depending on the image format, sub-sampling may be performed on samplesneighbor the reconstructed block of the first color component and thecorresponding reconstructed block of the first color component. Forexample, when one sample of the second color component corresponds tofour samples of the first color component, one corresponding sample maybe calculated by performing sub-sampling on the four samples of thefirst color component. When sub-sampling is performed, derivation of theparameters for the linear model and inter-color intra prediction may beperformed based on the sub-sampled corresponding sample.

Information about whether inter-color intra prediction is performedand/or the range of the template may be signaled in an intra-predictionmode.

The target block may be partitioned into two or four sub-blocks in ahorizontal direction and/or a vertical direction.

The sub-blocks resulting from the partitioning may be sequentiallyreconstructed. That is, as intra-prediction is performed on eachsub-block, a sub-prediction block for the sub-block may be generated.Also, as dequantization (inverse quantization) and/or an inversetransform are performed on each sub-block, a sub-residual block for thecorresponding sub-block may be generated. A reconstructed sub-block maybe generated by adding the sub-prediction block to the sub-residualblock. The reconstructed sub-block may be used as a reference sample forintra prediction of the sub-block having the next priority.

A sub-block may be a block including a specific number (e.g., 16) ofsamples or more. For example, when the target block is an 8×4 block or a4×8 block, the target block may be partitioned into two sub-blocks.Also, when the target block is a 4×4 block, the target block cannot bepartitioned into sub-blocks. When the target block has another size, thetarget block may be partitioned into four sub-blocks.

Information about whether intra prediction based on such sub-blocks isperformed and/or information about a partition direction (horizontaldirection or vertical direction) may be signaled.

Such sub-block-based intra prediction may be limited such that it isperformed only when reference sample line 0 is used. Whensub-block-based intra-prediction is performed, filtering of a predictionblock, which will be described below, may not be performed.

A final prediction block may be generated by performing filtering on theprediction block generated via intra prediction.

Filtering may be performed by applying specific weights to a filteringtarget sample, which is the target to be filtered, a left referencesample, an above reference sample, and/or an above-left referencesample.

The weights and/or reference samples (e.g., the range of referencesamples, the locations of the reference samples, etc.) used forfiltering may be determined based on at least one of a block size, anintra-prediction mode, and the location of the filtering target samplein a prediction block.

For example, filtering may be performed only in a specificintra-prediction mode (e.g., DC mode, planar mode, vertical mode,horizontal mode, diagonal mode and/or adjacent diagonal mode).

The adjacent diagonal mode may be a mode having a number obtained byadding k to the number of the diagonal mode, and may be a mode having anumber obtained by subtracting k from the number of the diagonal mode.In other words, the number of the adjacent diagonal mode may be the sumof the number of the diagonal mode and k, or may be the differencebetween the number of the diagonal mode and k. For example, k may be apositive integer of 8 or less.

The intra-prediction mode of the target block may be derived using theintra-prediction mode of a neighbor block present near the target block,and such a derived intra-prediction mode may be entropy-encoded and/orentropy-decoded.

For example, when the intra-prediction mode of the target block isidentical to the intra-prediction mode of the neighbor block,information indicating that the intra-prediction mode of the targetblock is identical to the intra-prediction mode of the neighbor blockmay be signaled using specific flag information.

Further, for example, indicator information for a neighbor block havingan intra-prediction mode identical to the intra-prediction mode of thetarget block, among intra-prediction modes of multiple neighbor blocks,may be signaled.

For example, when the intra-prediction mode of the target block isdifferent from the intra-prediction mode of the neighbor block, entropyencoding and/or entropy decoding may be performed on information aboutthe intra-prediction mode of the target block by performing entropyencoding and/or entropy decoding based on the intra-prediction mode ofthe neighbor block.

FIG. 9 is a diagram for explaining an embodiment of an inter predictionprocedure.

The rectangles shown in FIG. 9 may represent images (or pictures).Further, in FIG. 9 , arrows may represent prediction directions. Anarrow pointing from a first picture to a second picture means that thesecond picture refers to the first picture. That is, each image may beencoded and/or decoded depending on the prediction direction.

Images may be classified into an Intra Picture (I picture), aUni-prediction Picture or Predictive Coded Picture (P picture), and aBi-prediction Picture or Bi-predictive Coded Picture (B picture)depending on the encoding type. Each picture may be encoded and/ordecoded depending on the encoding type thereof.

When a target image that is the target to be encoded is an I picture,the target image may be encoded using data contained in the image itselfwithout inter prediction that refers to other images. For example, an Ipicture may be encoded only via intra prediction.

When a target image is a P picture, the target image may be encoded viainter prediction, which uses reference pictures existing in onedirection. Here, the one direction may be a forward direction or abackward direction.

When a target image is a B picture, the image may be encoded via interprediction that uses reference pictures existing in two directions, ormay be encoded via inter prediction that uses reference picturesexisting in one of a forward direction and a backward direction. Here,the two directions may be the forward direction and the backwarddirection.

A P picture and a B picture that are encoded and/or decoded usingreference pictures may be regarded as images in which inter predictionis used.

Below, inter prediction in an inter mode according to an embodiment willbe described in detail.

Inter prediction or a motion compensation may be performed using areference image and motion information.

In an inter mode, the encoding apparatus 100 may perform interprediction and/or motion compensation on a target block. The decodingapparatus 200 may perform inter prediction and/or motion compensation,corresponding to inter prediction and/or motion compensation performedby the encoding apparatus 100, on a target block.

Motion information of the target block may be individually derived bythe encoding apparatus 100 and the decoding apparatus 200 during theinter prediction. The motion information may be derived using motioninformation of a reconstructed neighbor block, motion information of acol block, and/or motion information of a block adjacent to the colblock.

For example, the encoding apparatus 100 or the decoding apparatus 200may perform prediction and/or motion compensation by using motioninformation of a spatial candidate and/or a temporal candidate as motioninformation of the target block. The target block may mean a PU and/or aPU partition.

A spatial candidate may be a reconstructed block which is spatiallyadjacent to the target block.

A temporal candidate may be a reconstructed block corresponding to thetarget block in a previously reconstructed co-located picture (colpicture).

In inter prediction, the encoding apparatus 100 and the decodingapparatus 200 may improve encoding efficiency and decoding efficiency byutilizing the motion information of a spatial candidate and/or atemporal candidate. The motion information of a spatial candidate may bereferred to as ‘spatial motion information’. The motion information of atemporal candidate may be referred to as ‘temporal motion information’.

Below, the motion information of a spatial candidate may be the motioninformation of a PU including the spatial candidate. The motioninformation of a temporal candidate may be the motion information of aPU including the temporal candidate. The motion information of acandidate block may be the motion information of a PU including thecandidate block.

Inter prediction may be performed using a reference picture.

The reference picture may be at least one of a picture previous to atarget picture and a picture subsequent to the target picture. Thereference picture may be an image used for the prediction of the targetblock.

In inter prediction, a region in the reference picture may be specifiedby utilizing a reference picture index (or refIdx) for indicating areference picture, a motion vector, which will be described later, etc.Here, the region specified in the reference picture may indicate areference block.

Inter prediction may select a reference picture, and may also select areference block corresponding to the target block from the referencepicture. Further, inter prediction may generate a prediction block forthe target block using the selected reference block.

The motion information may be derived during inter prediction by each ofthe encoding apparatus 100 and the decoding apparatus 200.

A spatial candidate may be a block 1) which is present in a targetpicture, 2) which has been previously reconstructed via encoding and/ordecoding, and 3) which is adjacent to the target block or is located atthe corner of the target block. Here, the “block located at the cornerof the target block” may be either a block vertically adjacent to aneighbor block that is horizontally adjacent to the target block, or ablock horizontally adjacent to a neighbor block that is verticallyadjacent to the target block. Further, “block located at the corner ofthe target block” may have the same meaning as “block adjacent to thecorner of the target block”. The meaning of “block located at the cornerof the target block” may be included in the meaning of “block adjacentto the target block”.

For example, a spatial candidate may be a reconstructed block located tothe left of the target block, a reconstructed block located above thetarget block, a reconstructed block located at the below-left corner ofthe target block, a reconstructed block located at the above-rightcorner of the target block, or a reconstructed block located at theabove-left corner of the target block.

Each of the encoding apparatus 100 and the decoding apparatus 200 mayidentify a block present at the location spatially corresponding to thetarget block in a col picture. The location of the target block in thetarget picture and the location of the identified block in the colpicture may correspond to each other.

Each of the encoding apparatus 100 and the decoding apparatus 200 maydetermine a col block present at the predefined relative location forthe identified block to be a temporal candidate. The predefined relativelocation may be a location present inside and/or outside the identifiedblock.

For example, the col block may include a first col block and a secondcol block. When the coordinates of the identified block are (xP, yP) andthe size of the identified block is represented by (nPSW, nPSH), thefirst col block may be a block located at coordinates (xP+nPSW,yP+nPSH). The second col block may be a block located at coordinates(xP+(nPSW>>1), yP+(nPSH>>1)). The second col block may be selectivelyused when the first col block is unavailable.

The motion vector of the target block may be determined based on themotion vector of the col block. Each of the encoding apparatus 100 andthe decoding apparatus 200 may scale the motion vector of the col block.The scaled motion vector of the col block may be used as the motionvector of the target block. Further, a motion vector for the motioninformation of a temporal candidate stored in a list may be a scaledmotion vector.

The ratio of the motion vector of the target block to the motion vectorof the col block may be identical to the ratio of a first temporaldistance to a second temporal distance. The first temporal distance maybe the distance between the reference picture and the target picture ofthe target block. The second temporal distance may be the distancebetween the reference picture and the col picture of the col block.

The scheme for deriving motion information may change depending on theinter-prediction mode of a target block. For example, asinter-prediction modes applied for inter prediction, an Advanced MotionVector Predictor (AMVP) mode, a merge mode, a skip mode, a merge modewith a motion vector difference, a sub block merge mode, a trianglepartition mode, an inter-intra combined prediction mode, an affine intermode, a current picture reference mode, etc. may be present. The mergemode may also be referred to as a “motion merge mode”. Individual modeswill be described in detail below.

1) AMVP Mode

When an AMVP mode is used, the encoding apparatus 100 may search aneighbor region of a target block for a similar block. The encodingapparatus 100 may acquire a prediction block by performing prediction onthe target block using motion information of the found similar block.The encoding apparatus 100 may encode a residual block, which is thedifference between the target block and the prediction block.

1-1) Creation of List of Prediction Motion Vector Candidates

When an AMVP mode is used as the prediction mode, each of the encodingapparatus 100 and the decoding apparatus 200 may create a list ofprediction motion vector candidates using the motion vector of a spatialcandidate, the motion vector of a temporal candidate, and a zero vector.The prediction motion vector candidate list may include one or moreprediction motion vector candidates. At least one of the motion vectorof a spatial candidate, the motion vector of a temporal candidate, and azero vector may be determined and used as a prediction motion vectorcandidate.

Hereinafter, the terms “prediction motion vector (candidate)” and“motion vector (candidate)” may be used to have the same meaning, andmay be used interchangeably with each other.

Hereinafter, the terms “prediction motion vector candidate” and “AMVPcandidate” may be used to have the same meaning, and may be usedinterchangeably with each other.

Hereinafter, the terms “prediction motion vector candidate list” and“AMVP candidate list” may be used to have the same meaning, and may beused interchangeably with each other.

Spatial candidates may include a reconstructed spatial neighbor block.In other words, the motion vector of the reconstructed neighbor blockmay be referred to as a “spatial prediction motion vector candidate”.

Temporal candidates may include a col block and a block adjacent to thecol block. In other words, the motion vector of the col block or themotion vector of the block adjacent to the col block may be referred toas a “temporal prediction motion vector candidate”.

The zero vector may be a (0, 0) motion vector.

The prediction motion vector candidates may be motion vector predictorsfor predicting a motion vector. Also, in the encoding apparatus 100,each prediction motion vector candidate may be an initial searchlocation for a motion vector.

1-2) Search for Motion Vectors that Use List of Prediction Motion VectorCandidates

The encoding apparatus 100 may determine the motion vector to be used toencode a target block within a search range using a list of predictionmotion vector candidates. Further, the encoding apparatus 100 maydetermine a prediction motion vector candidate to be used as theprediction motion vector of the target block, among prediction motionvector candidates present in the prediction motion vector candidatelist.

The motion vector to be used to encode the target block may be a motionvector that can be encoded at minimum cost.

Further, the encoding apparatus 100 may determine whether to use theAMVP mode to encode the target block.

1-3) Transmission of Inter-Prediction Information

The encoding apparatus 100 may generate a bitstream includinginter-prediction information required for inter prediction. The decodingapparatus 200 may perform inter prediction on the target block using theinter-prediction information of the bitstream.

The inter-prediction information may contain 1) mode informationindicating whether an AMVP mode is used, 2) a prediction motion vectorindex, 3) a Motion Vector Difference (MVD), 4) a reference direction,and 5) a reference picture index.

Hereinafter, the terms “prediction motion vector index” and “AMVP index”may be used to have the same meaning, and may be used interchangeablywith each other.

Further, the inter-prediction information may contain a residual signal.

The decoding apparatus 200 may acquire a prediction motion vector index,an MVD, a reference direction, and a reference picture index from thebitstream through entropy decoding when mode information indicates thatthe AMVP mode is used.

The prediction motion vector index may indicate a prediction motionvector candidate to be used for the prediction of a target block, amongprediction motion vector candidates included in the prediction motionvector candidate list.

1-4) Inter Prediction in AMVP Mode that Uses Inter-PredictionInformation

The decoding apparatus 200 may derive prediction motion vectorcandidates using a prediction motion vector candidate list, and maydetermine the motion information of a target block based on the derivedprediction motion vector candidates.

The decoding apparatus 200 may determine a motion vector candidate forthe target block, among the prediction motion vector candidates includedin the prediction motion vector candidate list, using a predictionmotion vector index. The decoding apparatus 200 may select a predictionmotion vector candidate, indicated by the prediction motion vectorindex, from among prediction motion vector candidates included in theprediction motion vector candidate list, as the prediction motion vectorof the target block.

The encoding apparatus 100 may generate an entropy-encoded predictionmotion vector index by applying entropy encoding to a prediction motionvector index, and may generate a bitstream including the entropy-encodedprediction motion vector index. The entropy-encoded prediction motionvector index may be signaled from the encoding apparatus 100 to thedecoding apparatus 200 through a bitstream. The decoding apparatus 200may extract the entropy-encoded prediction motion vector index from thebitstream, and may acquire the prediction motion vector index byapplying entropy decoding to the entropy-encoded prediction motionvector index.

The motion vector to be actually used for inter prediction of the targetblock may not match the prediction motion vector. In order to indicatethe difference between the motion vector to be actually used for interprediction of the target block and the prediction motion vector, an MVDmay be used. The encoding apparatus 100 may derive a prediction motionvector similar to the motion vector to be actually used for interprediction of the target block so as to use an MVD that is as small aspossible.

A Motion Vector Difference (MVD) may be the difference between themotion vector of the target block and the prediction motion vector. Theencoding apparatus 100 may calculate the MVD, and may generate anentropy-encoded MVD by applying entropy encoding to the MVD. Theencoding apparatus 100 may generate a bitstream including theentropy-encoded MVD.

The MVD may be transmitted from the encoding apparatus 100 to thedecoding apparatus 200 through the bitstream. The decoding apparatus 200may extract the entropy-encoded MVD from the bitstream, and may acquirethe MVD by applying entropy decoding to the entropy-encoded MVD.

The decoding apparatus 200 may derive the motion vector of the targetblock by summing the MVD and the prediction motion vector. In otherwords, the motion vector of the target block derived by the decodingapparatus 200 may be the sum of the MVD and the motion vector candidate.

Also, the encoding apparatus 100 may generate entropy-encoded MVDresolution information by applying entropy encoding to calculated MVDresolution information, and may generate a bitstream including theentropy-encoded MVD resolution information. The decoding apparatus 200may extract the entropy-encoded MVD resolution information from thebitstream, and may acquire MVD resolution information by applyingentropy decoding to the entropy-encoded MVD resolution information. Thedecoding apparatus 200 may adjust the resolution of the MVD using theMVD resolution information.

Meanwhile, the encoding apparatus 100 may calculate an MVD based on anaffine model. The decoding apparatus 200 may derive the affine controlmotion vector of the target block through the sum of the MVD and anaffine control motion vector candidate, and may derive the motion vectorof a sub-block using the affine control motion vector.

The reference direction may indicate a list of reference pictures to beused for prediction of the target block. For example, the referencedirection may indicate one of a reference picture list L0 and areference picture list L1.

The reference direction merely indicates the reference picture list tobe used for prediction of the target block, and may not mean that thedirections of reference pictures are limited to a forward direction or abackward direction. In other words, each of the reference picture listL0 and the reference picture list L1 may include pictures in a forwarddirection and/or a backward direction.

That the reference direction is unidirectional may mean that a singlereference picture list is used. That the reference direction isbidirectional may mean that two reference picture lists are used. Inother words, the reference direction may indicate one of the case whereonly the reference picture list L0 is used, the case where only thereference picture list L1 is used, and the case where two referencepicture lists are used.

The reference picture index may indicate a reference picture that isused for prediction of the target block, among reference picturespresent in a reference picture list. The encoding apparatus 100 maygenerate an entropy-encoded reference picture index by applying entropyencoding to the reference picture index, and may generate a bitstreamincluding the entropy-encoded reference picture index. Theentropy-encoded reference picture index may be signaled from theencoding apparatus 100 to the decoding apparatus 200 through thebitstream. The decoding apparatus 200 may extract the entropy-encodedreference picture index from the bitstream, and may acquire thereference picture index by applying entropy decoding to theentropy-encoded reference picture index.

When two reference picture lists are used to predict the target block, asingle reference picture index and a single motion vector may be usedfor each of the reference picture lists. Further, when two referencepicture lists are used to predict the target block, two predictionblocks may be specified for the target block. For example, the (final)prediction block of the target block may be generated using the averageor weighted sum of the two prediction blocks for the target block.

The motion vector of the target block may be derived by the predictionmotion vector index, the MVD, the reference direction, and the referencepicture index.

The decoding apparatus 200 may generate a prediction block for thetarget block based on the derived motion vector and the referencepicture index. For example, the prediction block may be a referenceblock, indicated by the derived motion vector, in the reference pictureindicated by the reference picture index.

Since the prediction motion vector index and the MVD are encoded withoutthe motion vector itself of the target block being encoded, the numberof bits transmitted from the encoding apparatus 100 to the decodingapparatus 200 may be decreased, and encoding efficiency may be improved.

For the target block, the motion information of reconstructed neighborblocks may be used. In a specific inter-prediction mode, the encodingapparatus 100 may not separately encode the actual motion information ofthe target block. The motion information of the target block is notencoded, and additional information that enables the motion informationof the target block to be derived using the motion information ofreconstructed neighbor blocks may be encoded instead. As the additionalinformation is encoded, the number of bits transmitted to the decodingapparatus 200 may be decreased, and encoding efficiency may be improved.

For example, as inter-prediction modes in which the motion informationof the target block is not directly encoded, there may be a skip modeand/or a merge mode. Here, each of the encoding apparatus 100 and thedecoding apparatus 200 may use an identifier and/or an index thatindicates a unit, the motion information of which is to be used as themotion information of the target unit, among reconstructed neighborunits.

2) Merge Mode

As a scheme for deriving the motion information of a target block, thereis merging. The term “merging” may mean the merging of the motion ofmultiple blocks. “Merging” may mean that the motion information of oneblock is also applied to other blocks. In other words, a merge mode maybe a mode in which the motion information of the target block is derivedfrom the motion information of a neighbor block.

When a merge mode is used, the encoding apparatus 100 may predict themotion information of a target block using the motion information of aspatial candidate and/or the motion information of a temporal candidate.The spatial candidate may include a reconstructed spatial neighbor blockthat is spatially adjacent to the target block. The spatial neighborblock may include a left neighbor block and an above neighbor block. Thetemporal candidate may include a col block. The terms “spatialcandidate” and “spatial merge candidate” may be used to have the samemeaning, and may be used interchangeably with each other. The terms“temporal candidate” and “temporal merge candidate” may be used to havethe same meaning, and may be used interchangeably with each other.

The encoding apparatus 100 may acquire a prediction block viaprediction. The encoding apparatus 100 may encode a residual block,which is the difference between the target block and the predictionblock.

2-1) Creation of Merge Candidate List

When the merge mode is used, each of the encoding apparatus 100 and thedecoding apparatus 200 may create a merge candidate list using themotion information of a spatial candidate and/or the motion informationof a temporal candidate. The motion information may include 1) a motionvector, 2) a reference picture index, and 3) a reference direction. Thereference direction may be unidirectional or bidirectional. Thereference direction may mean a inter prediction indicator.

The merge candidate list may include merge candidates. The mergecandidates may be motion information. In other words, the mergecandidate list may be a list in which pieces of motion information arestored.

The merge candidates may be pieces of motion information of temporalcandidates and/or spatial candidates. In other words, the mergecandidates list may comprise motion information of a temporal candidatesand/or spatial candidates, etc.

Further, the merge candidate list may include new merge candidatesgenerated by a combination of merge candidates that are already presentin the merge candidate list. In other words, the merge candidate listmay include new motion information generated by a combination of piecesof motion information previously present in the merge candidate list.

Also, a merge candidate list may include history-based merge candidates.The history-based merge candidates may be the motion information of ablock which is encoded and/or decoded prior to a target block.

Also, a merge candidate list may include a merge candidate based on anaverage of two merge candidates.

The merge candidates may be specific modes deriving inter predictioninformation. The merge candidate may be information indicating aspecific mode deriving inter prediction information. Inter predictioninformation of a target block may be derived according to a specificmode which the merge candidate indicates. Furthermore, the specific modemay include a process of deriving a series of inter predictioninformation. This specific mode may be an inter prediction informationderivation mode or a motion information derivation mode.

The inter prediction information of the target block may be derivedaccording to the mode indicated by the merge candidate selected by themerge index among the merge candidates in the merge candidate list.

For example, the motion information derivation modes in the mergecandidate list may be at least one of 1) motion information derivationmode for a sub-block unit and 2) an affine motion information derivationmode.

Furthermore, the merge candidate list may include motion information ofa zero vector. The zero vector may also be referred to as a “zero-mergecandidate”.

In other words, pieces of motion information in the merge candidate listmay be at least one of 1) motion information of a spatial candidate, 2)motion information of a temporal candidate, 3) motion informationgenerated by a combination of pieces of motion information previouslypresent in the merge candidate list, and 4) a zero vector.

Motion information may include 1) a motion vector, 2) a referencepicture index, and 3) a reference direction. The reference direction mayalso be referred to as an “inter-prediction indicator”. The referencedirection may be unidirectional or bidirectional. The unidirectionalreference direction may indicate L0 prediction or L1 prediction.

The merge candidate list may be created before prediction in the mergemode is performed.

The number of merge candidates in the merge candidate list may bepredefined. Each of the encoding apparatus 100 and the decodingapparatus 200 may add merge candidates to the merge candidate listdepending on the predefined scheme and predefined priorities so that themerge candidate list has a predefined number of merge candidates. Themerge candidate list of the encoding apparatus 100 and the mergecandidate list of the decoding apparatus 200 may be made identical toeach other using the predefined scheme and the predefined priorities.

Merging may be applied on a CU basis or a PU basis. When merging isperformed on a CU basis or a PU basis, the encoding apparatus 100 maytransmit a bitstream including predefined information to the decodingapparatus 200. For example, the predefined information may contain 1)information indicating whether to perform merging for individual blockpartitions, and 2) information about a block with which merging is to beperformed, among blocks that are spatial candidates and/or temporalcandidates for the target block.

2-2) Search for Motion Vector that Uses Merge Candidate List

The encoding apparatus 100 may determine merge candidates to be used toencode a target block. For example, the encoding apparatus 100 mayperform prediction on the target block using merge candidates in themerge candidate list, and may generate residual blocks for the mergecandidates. The encoding apparatus 100 may use a merge candidate thatincurs the minimum cost in prediction and in the encoding of residualblocks to encode the target block.

Further, the encoding apparatus 100 may determine whether to use a mergemode to encode the target block.

2-3) Transmission of Inter-Prediction Information

The encoding apparatus 100 may generate a bitstream that includesinter-prediction information required for inter prediction. The encodingapparatus 100 may generate entropy-encoded inter-prediction informationby performing entropy encoding on inter-prediction information, and maytransmit a bitstream including the entropy-encoded inter-predictioninformation to the decoding apparatus 200. Through the bitstream, theentropy-encoded inter-prediction information may be signaled to thedecoding apparatus 200 by the encoding apparatus 100. The decodingapparatus 200 may extract entropy-encoded inter-prediction informationfrom the bitstream, and may acquire inter-prediction information byapplying entropy decoding to the entropy-encoded inter-predictioninformation.

The decoding apparatus 200 may perform inter prediction on the targetblock using the inter-prediction information of the bitstream.

The inter-prediction information may contain 1) mode informationindicating whether a merge mode is used, 2) a merge index and 3)correction information.

Further, the inter-prediction information may contain a residual signal.

The decoding apparatus 200 may acquire the merge index from thebitstream only when the mode information indicates that the merge modeis used.

The mode information may be a merge flag. The unit of the modeinformation may be a block. Information about the block may include modeinformation, and the mode information may indicate whether a merge modeis applied to the block.

The merge index may indicate a merge candidate to be used for theprediction of the target block, among merge candidates included in themerge candidate list. Alternatively, the merge index may indicate ablock with which the target block is to be merged, among neighbor blocksspatially or temporally adjacent to the target block.

The encoding apparatus 100 may select a merge candidate having thehighest encoding performance among the merge candidates included in themerge candidate list and set a value of the merge index to indicate theselected merge candidate.

Correction information may be information used to correct a motionvector. The encoding apparatus 100 may generate correction information.The decoding apparatus 200 may correct the motion vector of a mergecandidate selected by a merge index based on the correction information.

The correction information may include at least one of informationindicating whether correction is to be performed, correction directioninformation, and correction size information. A prediction mode in whichthe motion vector is corrected based on the signaled correctioninformation may be referred to as a “merge mode having a motion vectordifference”.

2-4) Inter Prediction of Merge Mode that Uses Inter-PredictionInformation

The decoding apparatus 200 may perform prediction on the target blockusing the merge candidate indicated by the merge index, among mergecandidates included in the merge candidate list.

The motion vector of the target block may be specified by the motionvector, reference picture index, and reference direction of the mergecandidate indicated by the merge index.

3) Skip Mode

A skip mode may be a mode in which the motion information of a spatialcandidate or the motion information of a temporal candidate is appliedto the target block without change. Also, the skip mode may be a mode inwhich a residual signal is not used. In other words, when the skip modeis used, a reconstructed block may be the same as a prediction block.

The difference between the merge mode and the skip mode lies in whetheror not a residual signal is transmitted or used. That is, the skip modemay be similar to the merge mode except that a residual signal is nottransmitted or used.

When the skip mode is used, the encoding apparatus 100 may transmitinformation about a block, the motion information of which is to be usedas the motion information of the target block, among blocks that arespatial candidates or temporal candidates, to the decoding apparatus 200through a bitstream. The encoding apparatus 100 may generateentropy-encoded information by performing entropy encoding on theinformation, and may signal the entropy-encoded information to thedecoding apparatus 200 through a bitstream. The decoding apparatus 200may extract entropy-encoded information from the bitstream, and mayacquire information by applying entropy decoding to the entropy-encodedinformation.

Further, when the skip mode is used, the encoding apparatus 100 may nottransmit other syntax information, such as an MVD, to the decodingapparatus 200. For example, when the skip mode is used, the encodingapparatus 100 may not signal a syntax element related to at least one ofan MVD, a coded block flag, and a transform coefficient level to thedecoding apparatus 200.

3-1) Creation of Merge Candidate List

The skip mode may also use a merge candidate list. In other words, amerge candidate list may be used both in the merge mode and in the skipmode. In this aspect, the merge candidate list may also be referred toas a “skip candidate list” or a “merge/skip candidate list”.

Alternatively, the skip mode may use an additional candidate listdifferent from that of the merge mode. In this case, in the followingdescription, a merge candidate list and a merge candidate may bereplaced with a skip candidate list and a skip candidate, respectively.

The merge candidate list may be created before prediction in the skipmode is performed.

3-2) Search for Motion Vector that Uses Merge Candidate List

The encoding apparatus 100 may determine the merge candidates to be usedto encode a target block. For example, the encoding apparatus 100 mayperform prediction on the target block using the merge candidates in amerge candidate list. The encoding apparatus 100 may use a mergecandidate that incurs the minimum cost in prediction to encode thetarget block.

Further, the encoding apparatus 100 may determine whether to use a skipmode to encode the target block.

3-3) Transmission of Inter-Prediction Information

The encoding apparatus 100 may generate a bitstream that includesinter-prediction information required for inter prediction. The decodingapparatus 200 may perform inter prediction on the target block using theinter-prediction information of the bitstream.

The inter-prediction information may include 1) mode informationindicating whether a skip mode is used, and 2) a skip index.

The skip index may be identical to the above-described merge index.

When the skip mode is used, the target block may be encoded withoutusing a residual signal. The inter-prediction information may notcontain a residual signal. Alternatively, the bitstream may not includea residual signal.

The decoding apparatus 200 may acquire a skip index from the bitstreamonly when the mode information indicates that the skip mode is used. Asdescribed above, a merge index and a skip index may be identical to eachother. The decoding apparatus 200 may acquire the skip index from thebitstream only when the mode information indicates that the merge modeor the skip mode is used.

The skip index may indicate the merge candidate to be used for theprediction of the target block, among the merge candidates included inthe merge candidate list.

3-4) Inter Prediction in Skip Mode that Uses Inter-PredictionInformation

The decoding apparatus 200 may perform prediction on the target blockusing a merge candidate indicated by a skip index, among the mergecandidates included in a merge candidate list.

The motion vector of the target block may be specified by the motionvector, reference picture index, and reference direction of the mergecandidate indicated by the skip index.

4) Current Picture Reference Mode

The current picture reference mode may denote a prediction mode thatuses a previously reconstructed region in a target picture to which atarget block belongs.

A motion vector for specifying the previously reconstructed region maybe used. Whether the target block has been encoded in the currentpicture reference mode may be determined using the reference pictureindex of the target block.

A flag or index indicating whether the target block is a block encodedin the current picture reference mode may be signaled by the encodingapparatus 100 to the decoding apparatus 200. Alternatively, whether thetarget block is a block encoded in the current picture reference modemay be inferred through the reference picture index of the target block.

When the target block is encoded in the current picture reference mode,the target picture may exist at a fixed location or an arbitrarylocation in a reference picture list for the target block.

For example, the fixed location may be either a location where a valueof the reference picture index is 0 or the last location.

When the target picture exists at an arbitrary location in the referencepicture list, an additional reference picture index indicating such anarbitrary location may be signaled by the encoding apparatus 100 to thedecoding apparatus 200.

5) Sub-Block Merge Mode

A sub-block merge mode may be a mode in which motion information isderived from the sub-block of a CU.

When the sub-block merge mode is applied, a sub-block merge candidatelist may be generated using the motion information of a co-locatedsub-block (col-sub-block) of a target sub-block (i.e., a sub-block-basedtemporal merge candidate) in a reference image and/or an affine controlpoint motion vector merge candidate.

6) Triangle Partition Mode

In a triangle partition mode, a target block may be partitioned in adiagonal direction, and sub-target blocks resulting from partitioningmay be generated. For each sub-target block, motion information of thecorresponding sub-target block may be derived, and a prediction samplefor each sub-target block may be derived using the derived motioninformation. A prediction sample for the target block may be derivedthrough a weighted sum of the prediction samples for the sub-targetblocks resulting from the partitioning.

7) Combination Inter-Intra Prediction Mode

The combination inter-intra prediction mode may be a mode in which aprediction sample for a target block is derived using a weighted sum ofa prediction sample generated via inter-prediction and a predictionsample generated via intra-prediction.

In the above-described modes, the decoding apparatus 200 mayautonomously correct derived motion information. For example, thedecoding apparatus 200 may search a specific area for motion informationhaving the minimum sum of Absolute Differences (SAD) based on areference block indicated by the derived motion information, and mayderive the found motion information as corrected motion information.

In the above-described modes, the decoding apparatus 200 may compensatefor the prediction sample derived via inter prediction using an opticalflow.

In the above-described AMVP mode, merge mode, skip mode, etc., motioninformation to be used for prediction of the target block may bespecified among pieces of motion information in a list using the indexinformation of the list.

In order to improve encoding efficiency, the encoding apparatus 100 maysignal only the index of an element that incurs the minimum cost ininter prediction of the target block, among elements in the list. Theencoding apparatus 100 may encode the index, and may signal the encodedindex.

Therefore, the above-described lists (i.e. the prediction motion vectorcandidate list and the merge candidate list) must be able to be derivedby the encoding apparatus 100 and the decoding apparatus 200 using thesame scheme based on the same data. Here, the same data may include areconstructed picture and a reconstructed block. Further, in order tospecify an element using an index, the order of the elements in the listmust be fixed.

FIG. 10 illustrates spatial candidates according to an embodiment.

In FIG. 10 , the locations of spatial candidates are illustrated.

The large block in the center of the drawing may denote a target block.Five small blocks may denote spatial candidates.

The coordinates of the target block may be (xP, yP), and the size of thetarget block may be represented by (nPSW, nPSH).

Spatial candidate A₀ may be a block adjacent to the below-left corner ofthe target block. A₀ may be a block that occupies pixels located atcoordinates (xP−1, yP+nPSH+1).

Spatial candidate A₁ may be a block adjacent to the left of the targetblock. A₁ may be a lowermost block, among blocks adjacent to the left ofthe target block. Alternatively, A₁ may be a block adjacent to the topof A₀. A₁ may be a block that occupies pixels located at coordinates(xP−1, yP+nPSH).

Spatial candidate B₀ may be a block adjacent to the above-right cornerof the target block. B₀ may be a block that occupies pixels located atcoordinates (xP+nPSW+1, yP−1).

Spatial candidate B₁ may be a block adjacent to the top of the targetblock. Bi may be a rightmost block, among blocks adjacent to the top ofthe target block. Alternatively, B₁ may be a block adjacent to the leftof B₀. B₁ may be a block that occupies pixels located at coordinates(xP+nPSW, yP−1).

Spatial candidate B₂ may be a block adjacent to the above-left corner ofthe target block. B₂ may be a block that occupies pixels located atcoordinates (xP−1, yP−1).

Determination of Availability of Spatial Candidate and TemporalCandidate

In order to include the motion information of a spatial candidate or themotion information of a temporal candidate in a list, it must bedetermined whether the motion information of the spatial candidate orthe motion information of the temporal candidate is available.

Hereinafter, a candidate block may include a spatial candidate and atemporal candidate.

For example, the determination may be performed by sequentially applyingthe following steps 1) to 4).

Step 1) When a PU including a candidate block is out of the boundary ofa picture, the availability of the candidate block may be set to“false”. The expression “availability is set to false” may have the samemeaning as “set to be unavailable”.

Step 2) When a PU including a candidate block is out of the boundary ofa slice, the availability of the candidate block may be set to “false”.When the target block and the candidate block are located in differentslices, the availability of the candidate block may be set to “false”.

Step 3) When a PU including a candidate block is out of the boundary ofa tile, the availability of the candidate block may be set to “false”.When the target block and the candidate block are located in differenttiles, the availability of the candidate block may be set to “false”.

Step 4) When the prediction mode of a PU including a candidate block isan intra-prediction mode, the availability of the candidate block may beset to “false”. When a PU including a candidate block does not use interprediction, the availability of the candidate block may be set to“false”.

FIG. 11 illustrates the order of addition of motion information ofspatial candidates to a merge list according to an embodiment.

As shown in FIG. 11 , when pieces of motion information of spatialcandidates are added to a merge list, the order of A₁, B₁, B₀, A₀, andB₂ may be used. That is, pieces of motion information of availablespatial candidates may be added to the merge list in the order of A₁,B₁, B₀, A₀, and B₂.

Method for Deriving Merge List in Merge Mode and Skip Mode

As described above, the maximum number of merge candidates in the mergelist may be set. The set maximum number is indicated by “N”. The setnumber may be transmitted from the encoding apparatus 100 to thedecoding apparatus 200. The slice header of a slice may include N. Inother words, the maximum number of merge candidates in the merge listfor the target block of the slice may be set by the slice header. Forexample, the value of N may be basically 5.

Pieces of motion information (i.e., merge candidates) may be added tothe merge list in the order of the following steps 1) to 4).

Step 1) Among spatial candidates, available spatial candidates may beadded to the merge list. Pieces of motion information of the availablespatial candidates may be added to the merge list in the orderillustrated in FIG. 10 . Here, when the motion information of anavailable spatial candidate overlaps other motion information alreadypresent in the merge list, the motion information may not be added tothe merge list. The operation of checking whether the correspondingmotion information overlaps other motion information present in the listmay be referred to in brief as an “overlap check”.

The maximum number of pieces of motion information that are added may beN.

Step 2) When the number of pieces of motion information in the mergelist is less than N and a temporal candidate is available, the motioninformation of the temporal candidate may be added to the merge list.Here, when the motion information of the available temporal candidateoverlaps other motion information already present in the merge list, themotion information may not be added to the merge list.

Step 3) When the number of pieces of motion information in the mergelist is less than N and the type of a target slice is “B”, combinedmotion information generated by combined bidirectional prediction(bi-prediction) may be added to the merge list.

The target slice may be a slice including a target block.

The combined motion information may be a combination of L0 motioninformation and L1 motion information. L0 motion information may bemotion information that refers only to a reference picture list L0. L1motion information may be motion information that refers only to areference picture list L1.

In the merge list, one or more pieces of L0 motion information may bepresent. Further, in the merge list, one or more pieces of L1 motioninformation may be present.

The combined motion information may include one or more pieces ofcombined motion information. When the combined motion information isgenerated, L0 motion information and L1 motion information, which are tobe used for generation, among the one or more pieces of L0 motioninformation and the one or more pieces of L1 motion information, may bepredefined. One or more pieces of combined motion information may begenerated in a predefined order via combined bidirectional prediction,which uses a pair of different pieces of motion information in the mergelist. One of the pair of different pieces of motion information may beL0 motion information and the other of the pair may be L1 motioninformation.

For example, combined motion information that is added with the highestpriority may be a combination of L0 motion information having a mergeindex of 0 and L1 motion information having a merge index of 1. Whenmotion information having a merge index of 0 is not L0 motioninformation or when motion information having a merge index of 1 is notL1 motion information, the combined motion information may be neithergenerated nor added. Next, the combined motion information that is addedwith the next priority may be a combination of L0 motion information,having a merge index of 1, and L1 motion information, having a mergeindex of 0. Subsequent detailed combinations may conform to othercombinations of video encoding/decoding fields.

Here, when the combined motion information overlaps other motioninformation already present in the merge list, the combined motioninformation may not be added to the merge list.

Step 4) When the number of pieces of motion information in the mergelist is less than N, motion information of a zero vector may be added tothe merge list.

The zero-vector motion information may be motion information for whichthe motion vector is a zero vector.

The number of pieces of zero-vector motion information may be one ormore. The reference picture indices of one or more pieces of zero-vectormotion information may be different from each other. For example, thevalue of the reference picture index of first zero-vector motioninformation may be 0. The value of the reference picture index of secondzero-vector motion information may be 1.

The number of pieces of zero-vector motion information may be identicalto the number of reference pictures in the reference picture list.

The reference direction of zero-vector motion information may bebidirectional. Both of the motion vectors may be zero vectors. Thenumber of pieces of zero-vector motion information may be the smallerone of the number of reference pictures in the reference picture list L0and the number of reference pictures in the reference picture list L1.Alternatively, when the number of reference pictures in the referencepicture list L0 and the number of reference pictures in the referencepicture list L1 are different from each other, a reference directionthat is unidirectional may be used for a reference picture index thatmay be applied only to a single reference picture list.

The encoding apparatus 100 and/or the decoding apparatus 200 maysequentially add the zero-vector motion information to the merge listwhile changing the reference picture index.

When zero-vector motion information overlaps other motion informationalready present in the merge list, the zero-vector motion informationmay not be added to the merge list.

The order of the above-described steps 1) to 4) is merely exemplary, andmay be changed. Further, some of the above steps may be omitteddepending on predefined conditions.

Method for Deriving Prediction Motion Vector Candidate List in AMVP Mode

The maximum number of prediction motion vector candidates in aprediction motion vector candidate list may be predefined. Thepredefined maximum number is indicated by N. For example, the predefinedmaximum number may be 2.

Pieces of motion information (i.e. prediction motion vector candidates)may be added to the prediction motion vector candidate list in the orderof the following steps 1) to 3).

Step 1) Available spatial candidates, among spatial candidates, may beadded to the prediction motion vector candidate list. The spatialcandidates may include a first spatial candidate and a second spatialcandidate.

The first spatial candidate may be one of A₀, A₁, scaled A₀, and scaledA₁. The second spatial candidate may be one of B₀, B₁, B₂, scaled B₀,scaled B₁, and scaled B₂.

Pieces of motion information of available spatial candidates may beadded to the prediction motion vector candidate list in the order of thefirst spatial candidate and the second spatial candidate. In this case,when the motion information of an available spatial candidate overlapsother motion information already present in the prediction motion vectorcandidate list, the motion information may not be added to theprediction motion vector candidate list. In other words, when the valueof N is 2, if the motion information of a second spatial candidate isidentical to the motion information of a first spatial candidate, themotion information of the second spatial candidate may not be added tothe prediction motion vector candidate list.

The maximum number of pieces of motion information that are added may beN.

Step 2) When the number of pieces of motion information in theprediction motion vector candidate list is less than N and a temporalcandidate is available, the motion information of the temporal candidatemay be added to the prediction motion vector candidate list. In thiscase, when the motion information of the available temporal candidateoverlaps other motion information already present in the predictionmotion vector candidate list, the motion information may not be added tothe prediction motion vector candidate list.

Step 3) When the number of pieces of motion information in theprediction motion vector candidate list is less than N, zero-vectormotion information may be added to the prediction motion vectorcandidate list.

The zero-vector motion information may include one or more pieces ofzero-vector motion information. The reference picture indices of the oneor more pieces of zero-vector motion information may be different fromeach other.

The encoding apparatus 100 and/or the decoding apparatus 200 maysequentially add pieces of zero-vector motion information to theprediction motion vector candidate list while changing the referencepicture index.

When zero-vector motion information overlaps other motion informationalready present in the prediction motion vector candidate list, thezero-vector motion information may not be added to the prediction motionvector candidate list.

The description of the zero-vector motion information, made above inconnection with the merge list, may also be applied to zero-vectormotion information. A repeated description thereof will be omitted.

The order of the above-described steps 1) to 3) is merely exemplary, andmay be changed. Further, some of the steps may be omitted depending onpredefined conditions.

FIG. 12 illustrates a transform and quantization process according to anexample.

As illustrated in FIG. 12 , quantized levels may be generated byperforming a transform and/or quantization process on a residual signal.

A residual signal may be generated as the difference between an originalblock and a prediction block. Here, the prediction block may be a blockgenerated via intra prediction or inter prediction.

The residual signal may be transformed into a signal in a frequencydomain through a transform procedure that is a part of a quantizationprocedure.

A transform kernel used for a transform may include various DCT kernels,such as Discrete Cosine Transform (DCT) type 2 (DCT-II) and DiscreteSine Transform (DST) kernels.

These transform kernels may perform a separable transform or atwo-dimensional (2D) non-separable transform on the residual signal. Theseparable transform may be a transform indicating that a one-dimensional(1D) transform is performed on the residual signal in each of ahorizontal direction and a vertical direction.

The DCT type and the DST type, which are adaptively used for a 1Dtransform, may include DCT-V, DCT-VIII, DST-I, and DST-VII in additionto DCT-II, as shown in each of the following Table 3 and the followingtable 4.

TABLE 3 Transform set Transform candidates 0 DST-VII, DCT-VIII 1DST-VII, DST-I 2 DST-VII, DCT-V

TABLE 4 Transform set Transform candidates 0 DST-VII, DCT-VIII, DST-I 1DST-VII, DST-I, DCT-VIII 2 DST-VII, DCT-V, DST-I

As shown in Table 3 and Table 4, when a DCT type or a DST type to beused for a transform is derived, transform sets may be used. Eachtransform set may include multiple transform candidates. Each transformcandidate may be a DCT type or a DST type.

The following Table 5 shows examples of a transform set to be applied toa horizontal direction and a transform set to be applied to a verticaldirection depending on intra-prediction modes.

TABLE 5 Intra-prediction mode 0 1 2 3 4 5 6 7 8 9 Vertical 2 1 0 1 0 1 01 0 1 transform set Horizontal 2 1 0 1 0 1 0 1 0 1 transform setIntra-prediction mode 10 11 12 13 14 15 16 17 18 19 Vertical 0 1 0 1 0 00 0 0 0 transform set Horizontal 0 1 0 1 2 2 2 2 2 2 transform setIntra-prediction mode 20 21 22 23 24 25 26 27 28 29 Vertical 0 0 0 1 0 10 1 0 1 transform set Horizontal 2 2 2 1 0 1 0 1 0 1 transform setIntra-prediction mode 30 31 32 33 34 35 36 37 38 39 Vertical 0 1 0 1 0 10 1 0 1 transform set Horizontal 0 1 0 1 0 1 0 1 0 1 transform setIntra-prediction mode 40 41 42 43 44 45 46 47 48 49 Vertical 0 1 0 1 0 12 2 2 2 transform set Horizontal 0 1 0 1 0 1 0 0 0 0 transform setIntra-prediction mode 50 51 52 53 54 55 56 57 58 59 Vertical 2 2 2 2 2 10 1 0 1 transform set Horizontal 0 0 0 0 0 1 0 1 0 1 transform setIntra-prediction mode 60 61 62 63 64 65 66 Vertical 0 1 0 1 0 1 0transform set Horizontal 0 1 0 1 0 1 0 transform set

In Table 5, numbers of vertical transform sets and horizontal transformsets that are to be applied to the horizontal direction of a residualsignal depending on the intra-prediction modes of the target block areindicated.

As exemplified in FIGS. 4 and 5 , transform sets to be applied to thehorizontal direction and the vertical direction may be predefineddepending on the intra-prediction mode of the target block. The encodingapparatus 100 may perform a transform and an inverse transform on theresidual signal using a transform included in the transform setcorresponding to the intra-prediction mode of the target block. Further,the decoding apparatus 200 may perform an inverse transform on theresidual signal using a transform included in the transform setcorresponding to the intra-prediction mode of the target block.

In the transform and inverse transform, transform sets to be applied tothe residual signal may be determined, as exemplified in Tables 3, 4,and 5, and may not be signaled. Transform indication information may besignaled from the encoding apparatus 100 to the decoding apparatus 200.The transform indication information may be information indicating whichone of multiple transform candidates included in the transform set to beapplied to the residual signal is used.

For example, when the size of the target block is 64×64 or less,transform sets, each having three transforms, may be configureddepending on the intra-prediction modes. An optimal transform method maybe selected from among a total of nine multiple transform methodsresulting from combinations of three transforms in a horizontaldirection and three transforms in a vertical direction. Through such anoptimal transform method, the residual signal may be encoded and/ordecoded, and thus coding efficiency may be improved.

Here, information indicating which one of transforms belonging to eachtransform set has been used for at least one of a vertical transform anda horizontal transform may be entropy-encoded and/or -decoded. Here,truncated unary binarization may be used to encode and/or decode suchinformation.

As described above, methods using various transforms may be applied to aresidual signal generated via intra prediction or inter prediction.

The transform may include at least one of a first transform and asecondary transform. A transform coefficient may be generated byperforming the first transform on the residual signal, and a secondarytransform coefficient may be generated by performing the secondarytransform on the transform coefficient.

The first transform may be referred to as a “primary transform”.Further, the first transform may also be referred to as an “AdaptiveMultiple Transform (AMT) scheme”. AMT may mean that, as described above,different transforms are applied to respective 1D directions (i.e. avertical direction and a horizontal direction).

A secondary transform may be a transform for improving energyconcentration on a transform coefficient generated by the firsttransform. Similar to the first transform, the secondary transform maybe a separable transform or a non-separable transform. Such anon-separable transform may be a Non-Separable Secondary Transform(NSST).

The first transform may be performed using at least one of predefinedmultiple transform methods. For example, the predefined multipletransform methods may include a Discrete Cosine Transform (DCT), aDiscrete Sine Transform (DST), a Karhunen-Loeve Transform (KLT), etc.

Further, a first transform may be a transform having various transformtypes depending on a kernel function that defines a Discrete CosineTransform (DCT) or a Discrete Sine Transform (DST).

For example, the transform type may be determined based at least oneof 1) a prediction mode of a target block (for example, one of an intraprediction and an inter prediction), 2) a size of a target block, 3) ashape of a target block, 4) an intra prediction mode of a target block,5) a component of a target block (for example, one of a luma componentan a chroma component), and 6) a partitioning type applied to a targetblock (for example, one of a Quad Tree, a Binary Tree and a TernaryTree).

For example, the first transform may include transforms, such as DCT-2,DCT-5, DCT-7, DST-7, DST-1, DST-8, and DCT-8 depending on the transformkernel presented in the following Table 6. In the following Table 6,various transform types and transform kernel functions for MultipleTransform Selection (MTS) are exemplified.

MTS may refer to the selection of combinations of one or more DCT and/orDST kernels so as to transform a residual signal in a horizontal and/orvertical direction.

TABLE 6 Transform type Transform kernel function T_(i)(j) DCT-2${T_{i}(j)} = {{\omega_{0} \cdot \sqrt{\frac{2}{N}} \cdot {\cos\left( \frac{\pi \cdot i \cdot \left( {{2j} + 1} \right)}{2N} \right)}}{where}}$$\omega_{0} = {\sqrt{\frac{2}{N}}\left( {i = 0} \right){or}1({otherwise})}$DST-7${T_{i}(j)} = {\sqrt{\frac{4}{{2N} + 1}} \cdot {\sin\left( \frac{\pi \cdot \left( {{2j} + 1} \right) \cdot \left( {j + 1} \right)}{{2N} + 1} \right)}}$DCT-5${T_{i}(j)} = {{\omega_{0} \cdot \omega_{1} \cdot \sqrt{\frac{2}{{2N} - 1}} \cdot {\cos\left( \frac{2{\pi \cdot i \cdot j}}{{2N} + 1} \right)}}{where}}$$\omega_{0/1} = {\sqrt{\frac{2}{N}}\left( {{i{or}j} = 0} \right){or}1({otherwise})}$DCT-8${T_{i}(j)} = {\sqrt{\frac{4}{{2N} + 1}} \cdot {\cos\left( \frac{\pi \cdot \left( {{2j} + 1} \right) \cdot \left( {{2j} + 1} \right)}{{4N} + 2} \right)}}$DST-1${T_{i}(j)} = {\sqrt{\frac{2}{N + 1}} \cdot {\sin\left( \frac{\pi \cdot \left( {i + 1} \right) \cdot \left( {j + 1} \right)}{N + 1} \right)}}$

In Table 6, i an j may be integer values that are equal to or greaterthan and are less than or equal to N−1.

The secondary transform may be performed on the transform coefficientgenerated by performing the first transform.

As in the first transform, transform sets may also be defined in asecondary transform. The methods for deriving and/or determining theabove-described transform sets may be applied not only to the firsttransform but also to the secondary transform.

The first transform and the secondary transform may be determined for aspecific target.

For example, a first transform and a secondary transform may be appliedto signal components corresponding to one or more of a luminance (luma)component and a chrominance (chroma) component. Whether to apply thefirst transform and/or the secondary transform may be determineddepending on at least one of coding parameters for a target block and/ora neighbor block. For example, whether to apply the first transformand/or the secondary transform may be determined depending on the sizeand/or shape of the target block.

In the encoding apparatus 100 and the decoding apparatus 200, transforminformation indicating the transform method to be used for the targetmay be derived by utilizing specified information.

For example, the transform information may include a transform index tobe used for a primary transform and/or a secondary transform.Alternatively, the transform information may indicate that a primarytransform and/or a secondary transform are not used.

For example, when the target of a primary transform and a secondarytransform is a target block, the transform method(s) to be applied tothe primary transform and/or the secondary transform indicated by thetransform information may be determined depending on at least one ofcoding parameters for the target block and/or blocks neighbor the targetblock.

Alternatively, transform information indicating a transform method for aspecific target may be signaled from the encoding apparatus 100 to thedecoding apparatus 200.

For example, for a single CU, whether to use a primary transform, anindex indicating the primary transform, whether to use a secondarytransform, and an index indicating the secondary transform may bederived as the transform information by the decoding apparatus 200.Alternatively, for a single CU, the transform information, whichindicates whether to use a primary transform, an index indicating theprimary transform, whether to use a secondary transform, and an indexindicating the secondary transform, may be signaled.

The quantized transform coefficient (i.e. the quantized levels) may begenerated by performing quantization on the result, generated byperforming the first transform and/or the secondary transform, or on theresidual signal.

FIG. 13 illustrates diagonal scanning according to an example.

FIG. 14 illustrates horizontal scanning according to an example.

FIG. 15 illustrates vertical scanning according to an example.

Quantized transform coefficients may be scanned via at least one of(up-right) diagonal scanning, vertical scanning, and horizontal scanningdepending on at least one of an intra-prediction mode, a block size, anda block shape. The block may be a Transform Unit (TU).

Each scanning may be initiated at a specific start point, and may beterminated at a specific end point.

For example, quantized transform coefficients may be changed to 1Dvector forms by scanning the coefficients of a block using diagonalscanning of FIG. 13 . Alternatively, horizontal scanning of FIG. 14 orvertical scanning of FIG. 15 , instead of diagonal scanning, may be useddepending on the size and/or intra-prediction mode of a block.

Vertical scanning may be the operation of scanning 2D block-typecoefficients in a column direction. Horizontal scanning may be theoperation of scanning 2D block-type coefficients in a row direction.

In other words, which one of diagonal scanning, vertical scanning, andhorizontal scanning is to be used may be determined depending on thesize and/or inter-prediction mode of the block.

As illustrated in FIGS. 13, 14, and 15 , the quantized transformcoefficients may be scanned along a diagonal direction, a horizontaldirection or a vertical direction.

The quantized transform coefficients may be represented by block shapes.Each block may include multiple sub-blocks. Each sub-block may bedefined depending on a minimum block size or a minimum block shape.

In scanning, a scanning sequence depending on the type or direction ofscanning may be primarily applied to sub-blocks. Further, a scanningsequence depending on the direction of scanning may be applied toquantized transform coefficients in each sub-block.

For example, as illustrated in FIGS. 13, 14, and 15 , when the size of atarget block is 8×8, quantized transform coefficients may be generatedthrough a first transform, a secondary transform, and quantization onthe residual signal of the target block. Therefore, one of three typesof scanning sequences may be applied to four 4×4 sub-blocks, andquantized transform coefficients may also be scanned for each 4×4sub-block depending on the scanning sequence.

The encoding apparatus 100 may generate entropy-encoded quantizedtransform coefficients by performing entropy encoding on scannedquantized transform coefficients, and may generate a bitstream includingthe entropy-encoded quantized transform coefficients.

The decoding apparatus 200 may extract the entropy-encoded quantizedtransform coefficients from the bitstream, and may generate quantizedtransform coefficients by performing entropy decoding on theentropy-encoded quantized transform coefficients. The quantizedtransform coefficients may be aligned in the form of a 2D block viainverse scanning. Here, as the method of inverse scanning, at least oneof up-right diagonal scanning, vertical scanning, and horizontalscanning may be performed.

In the decoding apparatus 200, dequantization may be performed on thequantized transform coefficients. A secondary inverse transform may beperformed on the result generated by performing dequantization dependingon whether to perform the secondary inverse transform. Further, a firstinverse transform may be performed on the result generated by performingthe secondary inverse transform depending on whether the first inversetransform is to be performed. A reconstructed residual signal may begenerated by performing the first inverse transform on the resultgenerated by performing the secondary inverse transform.

For a luma component which is reconstructed via intra prediction orinter prediction, inverse mapping having a dynamic range may beperformed before in-loop filtering.

The dynamic range may be divided into 16 equal pieces, and mappingfunctions for respective pieces may be signaled. Such a mapping functionmay be signaled at a slice level or a tile group level.

An inverse mapping function for performing inverse mapping may bederived based on the mapping function.

In-loop filtering, the storage of a reference picture, and motioncompensation may be performed in an inverse mapping area.

A prediction block generated via inter prediction may be changed to amapped area through mapping using a mapping function, and the changedprediction block may be used to generate a reconstructed block. However,since intra prediction is performed in the mapped area, a predictionblock generated via intra prediction may be used to generate areconstructed block without requiring mapping and/or inverse mapping.

For example, when the target block is a residual block of a chromacomponent, the residual block may be changed to an inversely mapped areaby scaling the chroma component of the mapped area.

Whether scaling is available may be signaled at a slice level or a tilegroup level.

For example, scaling may be applied only to the case where mapping isavailable for a luma component and where the partitioning of the lumacomponent and the partitioning of the chroma component follow the sametree structure.

Scaling may be performed based on the average of the values of samplesin a luma prediction block, which corresponds to a chroma predictionblock. Here, when the target block uses inter prediction, the lumaprediction block may mean a mapped luma prediction block.

A value required for scaling may be derived by referring to a look-uptable using the index of a piece to which the average of sample valuesof the luma prediction block belongs.

The residual block may be changed to an inversely mapped area by scalingthe residual block using a finally derived value. Thereafter, for theblock of a chroma component, reconstruction, intra prediction, interprediction, in-loop filtering, and the storage of a reference picturemay be performed in the inversely mapped area.

For example, information indicating whether the mapping and/or inversemapping of a luma component and a chroma component are available may besignaled through a sequence parameter set.

A prediction block for the target block may be generated based on ablock vector. The block vector may indicate displacement between thetarget block and a reference block. The reference block may be a blockin a target image.

In this way, a prediction mode in which the prediction block isgenerated by referring to the target image may be referred to as an“Intra-Block Copy (IBC) mode”.

An IBC mode may be applied to a CU having a specific size. For example,the IBC mode may be applied to an M×N CU. Here, M and N may be less thanor equal to 64.

The IBC mode may include a skip mode, a merge mode, an AMVP mode, etc.In the case of the skip mode or the merge mode, a merge candidate listmay be configured, and a merge index is signaled, and thus a singlemerge candidate may be specified among merge candidates present in themerge candidate list. The block vector of the specified merge candidatemay be used as the block vector of the target block.

In the case of the AMVP mode, a differential block vector may besignaled. Also, a prediction block vector may be derived from the leftneighbor block and the above neighbor block of the target block.Further, an index indicating which neighbor block is to be used may besignaled.

A prediction block in the IBC mode may be included in a target CTU or aleft CTU, and may be limited to a block within a previouslyreconstructed area. For example, the value of a block vector may belimited so that a prediction block for a target block is located in aspecific area. The specific area may be an area defined by three 64×64blocks that are encoded and/or decoded prior to a 64×64 block includingthe target block. The value of the block vector is limited in this way,and thus memory consumption and device complexity caused by theimplementation of the IBC mode may be decreased.

FIG. 16 is a configuration diagram of an encoding apparatus according toan embodiment.

An encoding apparatus 1600 may correspond to the above-describedencoding apparatus 100.

The encoding apparatus 1600 may include a processing unit 1610, memory1630, a user interface (UI) input device 1650, a UI output device 1660,and storage 1640, which communicate with each other through a bus 1690.The encoding apparatus 1600 may further include a communication unit1620 coupled to a network 1699.

The processing unit 1610 may be a Central Processing Unit (CPU) or asemiconductor device for executing processing instructions stored in thememory 1630 or the storage 1640. The processing unit 1610 may be atleast one hardware processor.

The processing unit 1610 may generate and process signals, data orinformation that are input to the encoding apparatus 1600, are outputfrom the encoding apparatus 1600, or are used in the encoding apparatus1600, and may perform examination, comparison, determination, etc.related to the signals, data or information. In other words, inembodiments, the generation and processing of data or information andexamination, comparison and determination related to data or informationmay be performed by the processing unit 1610.

The processing unit 1610 may include an inter-prediction unit 110, anintra-prediction unit 120, a switch 115, a subtractor 125, a transformunit 130, a quantization unit 140, an entropy encoding unit 150, adequantization unit 160, an inverse transform unit 170, an adder 175, afilter unit 180, and a reference picture buffer 190.

At least some of the inter-prediction unit 110, the intra-predictionunit 120, the switch 115, the subtractor 125, the transform unit 130,the quantization unit 140, the entropy encoding unit 150, thedequantization unit 160, the inverse transform unit 170, the adder 175,the filter unit 180, and the reference picture buffer 190 may be programmodules, and may communicate with an external device or system. Theprogram modules may be included in the encoding apparatus 1600 in theform of an operating system, an application program module, or otherprogram modules.

The program modules may be physically stored in various types ofwell-known storage devices. Further, at least some of the programmodules may also be stored in a remote storage device that is capable ofcommunicating with the encoding apparatus 1200.

The program modules may include, but are not limited to, a routine, asubroutine, a program, an object, a component, and a data structure forperforming functions or operations according to an embodiment or forimplementing abstract data types according to an embodiment.

The program modules may be implemented using instructions or codeexecuted by at least one processor of the encoding apparatus 1600.

The processing unit 1610 may execute instructions or code in theinter-prediction unit 110, the intra-prediction unit 120, the switch115, the subtractor 125, the transform unit 130, the quantization unit140, the entropy encoding unit 150, the dequantization unit 160, theinverse transform unit 170, the adder 175, the filter unit 180, and thereference picture buffer 190.

A storage unit may denote the memory 1630 and/or the storage 1640. Eachof the memory 1630 and the storage 1640 may be any of various types ofvolatile or nonvolatile storage media. For example, the memory 1630 mayinclude at least one of Read-Only Memory (ROM) 1631 and Random AccessMemory (RAM) 1632.

The storage unit may store data or information used for the operation ofthe encoding apparatus 1600. In an embodiment, the data or informationof the encoding apparatus 1600 may be stored in the storage unit.

For example, the storage unit may store pictures, blocks, lists, motioninformation, inter-prediction information, bitstreams, etc.

The encoding apparatus 1600 may be implemented in a computer systemincluding a computer-readable storage medium.

The storage medium may store at least one module required for theoperation of the encoding apparatus 1600. The memory 1630 may store atleast one module, and may be configured such that the at least onemodule is executed by the processing unit 1610.

Functions related to communication of the data or information of theencoding apparatus 1600 may be performed through the communication unit1620.

For example, the communication unit 1620 may transmit a bitstream to adecoding apparatus 1600, which will be described later.

FIG. 17 is a configuration diagram of a decoding apparatus according toan embodiment.

The decoding apparatus 1700 may correspond to the above-describeddecoding apparatus 200.

The decoding apparatus 1700 may include a processing unit 1710, memory1730, a user interface (UI) input device 1750, a UI output device 1760,and storage 1740, which communicate with each other through a bus 1790.The decoding apparatus 1700 may further include a communication unit1720 coupled to a network 1799.

The processing unit 1710 may be a Central Processing Unit (CPU) or asemiconductor device for executing processing instructions stored in thememory 1730 or the storage 1740. The processing unit 1710 may be atleast one hardware processor.

The processing unit 1710 may generate and process signals, data orinformation that are input to the decoding apparatus 1700, are outputfrom the decoding apparatus 1700, or are used in the decoding apparatus1700, and may perform examination, comparison, determination, etc.related to the signals, data or information. In other words, inembodiments, the generation and processing of data or information andexamination, comparison and determination related to data or informationmay be performed by the processing unit 1710.

The processing unit 1710 may include an entropy decoding unit 210, adequantization unit 220, an inverse transform unit 230, anintra-prediction unit 240, an inter-prediction unit 250, a switch 245,an adder 255, a filter unit 260, and a reference picture buffer 270.

At least some of the entropy decoding unit 210, the dequantization unit220, the inverse transform unit 230, the intra-prediction unit 240, theinter-prediction unit 250, the adder 255, the switch 245, the filterunit 260, and the reference picture buffer 270 of the decoding apparatus200 may be program modules, and may communicate with an external deviceor system. The program modules may be included in the decoding apparatus1700 in the form of an operating system, an application program module,or other program modules.

The program modules may be physically stored in various types ofwell-known storage devices. Further, at least some of the programmodules may also be stored in a remote storage device that is capable ofcommunicating with the decoding apparatus 1700.

The program modules may include, but are not limited to, a routine, asubroutine, a program, an object, a component, and a data structure forperforming functions or operations according to an embodiment or forimplementing abstract data types according to an embodiment.

The program modules may be implemented using instructions or codeexecuted by at least one processor of the decoding apparatus 1700.

The processing unit 1710 may execute instructions or code in the entropydecoding unit 210, the dequantization unit 220, the inverse transformunit 230, the intra-prediction unit 240, the inter-prediction unit 250,the switch 245, the adder 255, the filter unit 260, and the referencepicture buffer 270.

A storage unit may denote the memory 1730 and/or the storage 1740. Eachof the memory 1730 and the storage 1740 may be any of various types ofvolatile or nonvolatile storage media. For example, the memory 1730 mayinclude at least one of ROM 1731 and RAM 1732.

The storage unit may store data or information used for the operation ofthe decoding apparatus 1700. In an embodiment, the data or informationof the decoding apparatus 1700 may be stored in the storage unit.

For example, the storage unit may store pictures, blocks, lists, motioninformation, inter-prediction information, bitstreams, etc.

The decoding apparatus 1700 may be implemented in a computer systemincluding a computer-readable storage medium.

The storage medium may store at least one module required for theoperation of the decoding apparatus 1700. The memory 1730 may store atleast one module, and may be configured such that the at least onemodule is executed by the processing unit 1710.

Functions related to communication of the data or information of thedecoding apparatus 1700 may be performed through the communication unit1720.

For example, the communication unit 1720 may receive a bitstream fromthe encoding apparatus 1700.

Concept of Picture, Slice, Tile, and Brick, and Configuration of Slice

FIG. 18 illustrates the division of a picture in a raster-scan slicemode according to an example.

One picture may be divided into one or more regions. A region may be atile.

For example, each region may have the shape of a rectangle. In otherwords, one or more rectangular regions may constitute one picture.

Alternatively, each region may have a shape in which more than one ormore rectangles are combined with each other. For example, the more thanone or more combined rectangles may be consecutive units following araster-scan order.

In an embodiment, a rectangle may be limited to a square.

The unit of division may be any of a slice, a tile, and a coding treeunit (CTU).

In FIG. 18 , each slice is indicated by a bold solid line. Each tile isindicated by a thin solid line. Each CTU is indicated by a dotted line.

A picture may be divided into slices and tiles.

The picture may be divided into one or more slices. Alternatively, thepicture may include one or more slices.

Information for division into one or more slices may be signaled througha network abstraction layer (NAL) unit. A NAL unit may include a sliceheader and slice data.

In FIG. 18 , the picture is divided into three slices.

The picture may be divided into one or more tiles. In the division ofthe picture into one or more tiles, the number of rows and the number ofcolumns corresponding to one or more tiles may be used. In other words,the picture may be divided into one or more tiles depending on n₁ andm₁. n₁ denotes the number of one or more rows corresponding to one ormore tiles. m₁ denotes the number of one or more columns correspondingto one or more tiles.

In FIG. 18 , the picture is divided into 12 tiles. A first slice of thepicture includes two tiles. A second slice of the picture includes fivetiles. A third slice of the picture includes five tiles.

The picture may be divided into one or more Coding Tree Units (CTUs).

Each tile may include one or more CTUs. Alternatively, each tile may bedivided into one or more CTUs. A tile may cover a region of arectangular in a picture. A tile may be composed of a sequence of CTUs.

Each tile may be composed of one or more CTUs corresponding to n₂ rowsand m₂ columns. n₂ may be an integer of 1 or more, and m₂ may be aninteger of 1 or more. Further, the picture may be composed of a set ofone or more CTUs included in the rectangular region of each tile.

Each CTU may occupy a rectangular region in a picture. Processing of oneor more CTUs in a tile may be performed using a raster-scan mode(raster-scan slice mode). Here, processing may include encoding,decoding, writing, and reading.

In FIG. 18 , each tile includes 18 CTUs. The 18 CTUs may be configuredin three rows and six columns.

At least a part of encoding of a slice and/or a tile and at least a partof decoding thereof may be independently performed on an additionalslice and/or an additional tile. For example, encoding may includeprediction, transform, quantization, entropy coding, dequantization,inverse transform, and reconstruction. Decoding may includedequantization, inverse transform, prediction, and reconstruction.

By utilizing these features, a slice and a tile may be used for parallelprocessing, which becomes necessary as the encoding apparatus 1600 andthe decoding apparatus 1700 become more complicated, and may be used toprovide a Region Of Interest (ROI) in the picture.

Each slice may be composed of one or more tiles of a picture.Alternatively, each slice may be composed of a sequence of one or moreCTUs comprising a tile. For example, each slice may include one or moreinteger number of tiles of the picture. Alternatively, each slice mayinclude one or more integer number of CTUs of the picture.

Each vertical slice boundary is always also a vertical tile boundary. Inother words, a vertical boundary of the slice can be constrained tocoincide with a boundary of the tile.

It is possible that a horizontal boundary of a slice is not a tileboundary. A horizontal boundary of a slice may be composed of horizontalCTU boundaries within a tile.

When a tile is partitioned into a plurality of rectangular slices, eachslice of the plurality of rectangular slices may be composed of aninteger number of consecutive complete CTU rows within the tile.

Each slice in the picture may be defined in two types of modes. A firstmode may be a raster-scan slice mode. A second mode may be a rectangularslice mode.

In the raster-scan slice mode, one slice may include the sequence ofcomplete tiles in a tile raster scan of the picture.

In the rectangular slice mode, one slice may include 1) multiplecomplete tiles which collectively form a rectangular region of thepicture, or 2) multiple consecutive complete CTU rows of one tile. Themultiple consecutive complete CTU rows of one tile may collectively formthe rectangular region of the picture.

One or more tiles in a rectangular slice may be processed in araster-scanning order.

Each sub-picture may include one or more slices which collectively covera rectangular region of a picture. The boundary of each sub-picture mayalways be the boundary of a slice. Further, the boundary of eachvertical sub-picture may always be the boundary of a vertical tile.

FIG. 19 illustrates the division of a picture in a rectangular slicemode according to an example.

In FIG. 19 , each slice is indicated by a bold solid line. Each tile isindicated by a thin solid line. Each CTU is indicated by a dotted line.

In FIG. 19 , the picture is divided into 24 tiles and nine rectangularslices. The 24 tiles are configured in six columns and four rows.

As illustrated in FIGS. 18 and 19 , each slice may include multipletiles.

FIG. 20 illustrates tiles and rectangular slices in a picture accordingto an example.

In FIG. 20 , each slice is indicated by a bold solid line. Each tile isindicated by a thin solid line. Each CTU is indicated by a dotted line.

The picture may be divided into one or more tiles and one or morerectangular slices.

In FIG. 20 , the picture is divided into four tiles. Further, thepicture is divided into four rectangular slices.

The picture may be encoded using three different color planes. A colorplane identifier for each slice may indicate the color plane of thecorresponding slice. Here, the slice may include only a CTU in a colorcorresponding to the color plane identifier of the slice. Each colorarray may be composed of slices having the same color plane identifier.

FIG. 21 illustrates the division of a picture into sub-picturesaccording to an example.

In FIG. 21 , each sub-picture is indicated by a bold dotted line. Eachslice is indicated by a bold solid line. Each tile is indicated by athin solid line. Each CTU is indicated by a thin dotted line.

In FIG. 21 , the picture may include 15 tiles. Each of the tiles mayinclude 4×4 CTUs. Also, the picture may be divided into 24 slices and 24sub-pictures. As described above with reference to FIGS. 18 to 20 , eachslice may be composed of one or more tiles, and may be composed of oneor more CTUs in the tiles.

Hereinafter, a target picture denotes a picture that is the target ofcurrent encoding/decoding. A target sub-picture denotes a sub-picturethat is the target of current encoding/decoding. A target slice denotesa slice that is the target of current encoding/decoding. A target blockmay be a block that is the target of current encoding/decoding. A targetpicture may include a target sub-picture. A target sub-picture mayinclude a target slice. A target slice may include a target block.

Configuration of Subpicture

As described above, a picture may be composed of one or moresubpictures.

In order to define one or more subpictures in the picture, a layout andidentifiers (ID) of the one or more subpictures may be required.

1) The layout and subpicture IDs may be signaled through a SequenceParameter Set (SPS).

The layout may be information for specifying the locations and areas ofone or more subpictures in the picture.

The widths and heights of respective subpictures may be signaled throughthe SPS. The unit of width and height may be a Coding Tree Unit (CTU).The width and height of each subpicture may be signaled in units of aCTU (or a CTU size).

The subpicture IDs of respective subpictures may be signaled through theSPS.

1) The layout and subpicture IDs may be signaled through a PictureParameter Set (PPS).

The subpicture IDs of respective subpictures may be signaled through thePPS.

The configuration of rectangular slices may be signaled through the PPS.

The configuration of rectangular slices may indicate 1) the number ofone or more rectangular slices and 2) the area occupied by each of theone or more rectangular slices.

Determination of Subpicture ID of Subpicture

FIG. 22 illustrates code for subpicture ID mapping according to anexample.

Subpicture ID mapping may be configured to determine the subpicture IDof each of one or more subpictures in a picture.

When the PPS is parsed, the syntax element “SubpicIdVal[i]” may bederived based on the code illustrated in FIG. 22 .

SubpicIdVal[i] may be the subpicture ID of an i-th subpicture, among theone or more subpictures in the picture. “i” may be equal to or greaterthan ‘0’ and less than or equal to ‘sps_num_subpics_minus1’.

The syntax element “sps_num_subpics_minus1+1” may indicate the number ofone or more subpictures in each picture. sps_num_subpics_minus1 may besignaled through the SPS.

The syntax element “subpic_id_mapping_explicitly_signalled_flag” may beinformation explicitly indicating whether subpicture ID mapping is to beperformed in the SPS or the PPS.

The syntax element “subpic_id_mapping_in_pps_flag” may be information(or a flag) indicating whether information about subpicture ID mappingis to be signaled through the PPS. When the information about subpictureID mapping is not signaled through the PPS, the information aboutsubpicture ID mapping may be signaled through the SPS.

The syntax element “pps_subpic_id[i]” may specify the subpicture ID ofthe i-th subpicture, among the one or more subpictures in the picture.

The syntax element “pps_subpic_id[i]” may be signaled through the PPS.

The syntax element “sps_subpic_id[i]” may specify the subpicture ID ofthe i-th subpicture, among the one or more subpictures in the picture.

The syntax element “sps_subpic_id[i]” may be signaled through the SPS.

Each picture may include one or more slices.

The slice header of each of the one or more slices may includeslice_subpic_id (or sh_subpic_id). The slice_subpic_id of each slice maybe signaled through the slice header.

slice_subpic_id may indicate the subpicture ID of a subpicture includinga slice.

If there is slice_subpic_id, the value of the variable “CurrSubpicIdx”may be derived such that SubpicIdVal[CurrSubpicIdx] is identical tosh_subpic_id. If there is no slice_subpic_id, the value of CurrSubpicIdxmay be derived as ‘0’.

The variable “CurrSubpicIdx” may be used to derive the location of atarget subpicture.

FIG. 23 illustrates signaling of slice addresses in a raster-scan slicemode according to an example.

A PPS may include the syntax element “rect_slice_flag”. The syntaxelement “rect_slice_flag” may specify the slice mode used for eachpicture that refers to the PPS.

When the value of pps_rect_slice_flag is ‘0’, a raster-scan slice modemay be used for each picture that refers to the PPS, and a layout ofslices may not be signaled in the PPS.

When the value of pps_rect_slice_flag is ‘1’, a rectangular slice modemay be used for each picture that refers to the PPS, and the layout ofslices may be signaled in the PPS.

Depending on the slice mode, a scheme for determining slice addressesmay differ.

In FIG. 23 , signaling of slice addresses performed when the value ofrect_slice_flag is ‘0’ is illustrated.

When the value of rect_slice_flag is ‘0’, the slice address of eachslice may be the raster-scan tile index of the first tile of the slice.That is, the slice address may be determined by units of tiles.

The slice addresses of one or more slices of the subpicture may rangefrom ‘0’ to ‘NumTilesInPic−1’.

In the example of FIG. 23 , the value of rect_slice may be ‘0’.

Further, NumofSlice (i.e., the number of one or more slices) may be 2.

Also, slice address [1] (i.e., the address of a first slice) may be ‘1’.Also, slice address [2] (i.e., the address of a second slice) may be‘4’.

Further, the range K of the slice address [i] may be defined as therange of integers from ‘0’ to ‘NumTileInPic−1’. NumTileInPic may be thenumber of one or more tiles in a picture.

FIG. 24 illustrates signaling of slice addresses in a rectangular slicemode according to an example.

In FIG. 24 , signaling of slice addresses performed when the value ofrect_slice_flag is ‘1’ is illustrated.

When the value of rect_slice_flag is ‘1’, the slice address of eachslice may be a slice index. Here, the slice address may be determined atthe level of a subpicture. That is, different slice addresses may berespectively assigned to one or more slices in a subpicture.

Since the slice addresses are determined at the level of the subpicture,the range of slice addresses may be defined as the range of integersfrom ‘0’ to ‘NumSliceInSubPic[CurrSubpicIdx]−1’. CurrSubpicIdx may bethe subpicture index of a target subpicture. NumSliceInSubPic[i] may bethe number of one or more slices in an i-th subpicture.

In the example of FIG. 24 , one or more slices may be present in asubpicture having a subpicture ID of 2.

Further, rect_slice may be ‘1’.

Furthermore, the syntax element “NumSlicesInSubpic[CurrSubPicIdx]”(i.e., the number of one or more slices in the target subpicture) may be‘2’.

Also, slice address [1] (the address of a first slice) may be ‘1’. Also,slice address [2] (the address of a second slice) may be ‘4’.

Further, the range K of the slice address[i] may be defined as the rangeof integers from ‘0’ to ‘NumSlicesInSubpic[CurrSubpicIdx]−1’.NumSlicesInSubpic[CurrSubpicIdx] may be the number of one or more slicesin the target subpicture.

FIG. 25 illustrates a part of the structure of a sequence parameter setaccording to an example.

The code of FIG. 25 shows a method for signaling a subpicture ID throughan SPS.

As the method for signaling the subpicture ID, 1) a method for signalinga subpicture ID through an SPS and 2) a method for signaling asubpicture ID through a PPS may be used.

When the SPS is used to signal a subpicture ID, 1) the subpicture ID maybe directly signaled through the SPS. 2) Alternatively, the subpictureID of the subpicture may be derived through the subpicture index of thesubpicture.

Here, the subpicture index may be equal to or greater than ‘0’ and lessthan or equal to ‘sps_num_subpics_minus1’.

Depending on a specific scan type, respective subpicture indexes may besequentially assigned to one or more subpictures. The scan type may be araster scan, a vertical scan, a horizontal scan, a z scan, a diagonalscan or the like.

When the SPS is not used to signal the subpicture ID of the subpicture,a PPS may be used to signal the subpicture ID of the subpicture.

Depending on the layout of one or more subpictures defined in the SPS,subpicture indices of the one or more subpictures may be the same forrespective pictures. In contrast, subpicture IDs of one or moresubpictures may be different for each picture.

The subpicture IDs of multiple subpictures constituting one picture maybe different from each other. Further, the subpicture ID of at least oneof the multiple subpictures constituting one picture may be identical tothat of other subpictures. Alternatively, all subpicture IDs of multiplesubpictures may be identical to each other.

As illustrated in FIG. 25 , when the value ofsubpic_id_mapping_explicitly_signalled_flag is ‘1’, each subpicture IDmay be signaled through an SPS.

Further, when the value of subpic_id_mapping_explicitly_signalled_flagis ‘0’, each subpicture ID may be derived using code, which will bedescribed later with reference to FIG. 26 .

Furthermore, even if the value ofsubpic_id_mapping_explicitly_signalled_flag is ‘1’, some subpicture IDsmay not be explicitly signaled, and subpicture IDs may be derived as aspecific value depending on specific schemes and specific conditions.Here, derivation may be performed based on at least one of the locationsof subpictures, the number of subpictures, and previously allocatedsubpicture IDs. Alternatively, the specific value may be determinedbased on at least one of the locations of subpictures, the number ofsubpictures, and previously allocated subpicture IDs.

The syntax element “subpic_id_mapping_in_sps_flag” may be informationindicating whether subpicture ID mapping is to be signaled in an SPSwhen the value of subpic_id_mapping_explicitly_signalled_flag is ‘1’.

When the value of subpic_id_mapping_explicitly_signalled_flag is ‘1’, ifthe value of subpic_id_mapping_in_sps_flag is ‘1’, subpicture ID mappingmay be signaled in the SPS.

As illustrated in FIG. 25 , when the value ofsubpic_id_mapping_explicitly_signalled_flag is ‘1’, if the value ofsubpic_id_mapping_in_sps_flag is ‘1’, sps_num_subpic_minus1+1 subpictureIDs (sps_subpic_id[i]) ranging from ‘0’ to ‘sps_num_subpic_minus1’ maybe signaled through the ‘for clause’.

When the value of subpic_id_mapping_explicitly_signalled_flag is ‘1’, ifthe value of subpic_id_mapping_in_sps_flag is ‘0’, subpicture ID mappingmay be signaled in a PPS. The PPS may be referred to by the codedpictures of a Coded Layer Video Sequence (CLVS).

FIG. 26 illustrates a method for deriving a subpicture ID according toan example.

The code of FIG. 26 may show a method of deriving the value ofsps_subpic_id when the value ofsubpic_id_mapping_explicitly_signalled_flag is ‘0’ and the value ofsubpic_infopresent_flag is ‘1’.

As illustrated in FIG. 26 , when the value ofsubpic_id_mapping_explicitly_signalled_flag is ‘0’ and the value ofsubpic_info_present_flag is ‘1’, the subpicture ID of a subpicturehaving a subpicture index of i may be ‘i’.

The subpicture may be divided into one or more slices. Therefore, whenthe slice address of each slice is derived, the location of a subpicturecorresponding to the subpicture ID may be derived using the derivedslice address.

FIG. 27 illustrates a scheme for utilizing a slice index for derivingthe address of a CTB in a slice according to an example.

The function “AddCtbToSlice” of FIG. 27 may add Coding Tree Blocks (CTB)to the slices.

In AddCtbToSlice, the CTB address of a corresponding CTB in a targetslice may be derived using the slice index sliceIdx.

CtbAddInSlice[i][j] may indicate the address of a j-th CTB in an i-thslice.

PicWidthInCtbs may be the width of a picture represented by units of aCTB.

NumCtusInSlice[i] may indicate the number of CTBs in the i-th slice.

FIG. 28 illustrates a scheme for utilizing a slice index for derivingthe address of a CTB in a target slice according to an example.

In the code of FIG. 28 , CTB addresses of CTBs in the target slice maybe derived using a slice address. Here, the CTB address of each of theCTBs in the target slice may be derived using the variable“CtbAddrInSlice”.

slice_address may indicate the slice index of the target slice (at thelevel of a subpicture).

CurrSubpicIdx may indicate the subpicture index of a target subpicture.

picLevelSliceIdx may be determined to indicate the slice index of thetarget slice at the picture level using slice_address andNumSlicesInSubpic[j]. In other words, picLevelSliceIdx may beinitialized to have the value of slice_address, and a number of slicesin subpictures preceding the target subpicture may be added topicLevelSliceIdx.

NumCtusInCurrSlice may represent the number of CTBs in the sliceindicated by picLevelSliceIdx.

Through the ‘for clause’ using NumCtusInCurrSlice,CtbAddrInSlice[picLevelSliceIdx][i] may be copied toCtbAddrInCurrSlice[i]. Through this copying, CTB addresses of one ormore CTBs in the target slice may be set.

Referring back to FIG. 25 , the code of FIG. 25 may represent a methodin which the subpicture ID of each subpicture is directly signaledthrough the SPS. Alternatively, the subpicture ID of each subpicture maybe derived through the index of the subpicture.

FIG. 29 illustrates a part of the structure of a Picture Parameter Set(PPS) according to an example.

The code of FIG. 29 may indicate a method for signaling a subpicture IDthrough a PPS.

As the method for signaling the subpicture ID, 1) a method for signalinga subpicture ID through an SPS and 2) a method for signaling asubpicture ID through a PPS may be used.

As illustrated in FIG. 29 , when the value ofsubpic_id_mapping_explicitly_signalled_pps_flag is ‘1’, the subpictureID may be signaled through the PPS.

Further, when the value ofsubpic_id_mapping_explicitly_signalled_pps_flag is ‘0’, the subpictureID may be derived using the code, described above with reference to FIG.26 .

The syntax element “subpic_id_mapping_in_pps_flag” may be informationindicating whether subpicture ID mapping is to be signaled in the PPSwhen the value of subpic_id_mapping_explicitly_signalled_pps_flag is‘1’.

When the value of subpic_id_mapping_explicitly_signalled_pps_flag is‘1’, if the value of subpic_id_mapping_in_pps_flag is ‘1’, subpicture IDmapping may be signaled in the PPS.

As illustrated in FIG. 29 , when the value ofsubpic_id_mapping_explicitly_signalled_pps_flag is ‘1’, if the value ofsubpic_id_mapping_in_pps_flag is 1, pps_num_subpic_minus1+1 PPSsubpicture IDs (pps_subpic_id[i]) ranging from ‘0’ to‘pps_num_subpic_minus1’ may be signaled through the ‘for clause’.

FIG. 30 illustrates code for subpicture ID mapping according to anexample.

For an SPS and a PPS, when the value ofsubpic_id_mapping_explicitly_signalled_flag is ‘1’ or when the value ofsubpic_id_mapping_explicitly_signalled_pps_flag is ‘1’, subpicture IDsmay be signaled through the SPS or PPS. When the value ofsubpic_id_mapping_explicitly_signalled_pps_flag is ‘1’, subpicture IDsmay be signaled through the PPS. When the value ofsubpic_id_mapping_explicitly_signalled_pps_flag is ‘0’, subpicture IDsmay be signaled through the SPS.

When the value of subpic_id_mapping_explicitly_signalled_flag is ‘0’ andthe value of subpic_id_mapping_explicitly_signalled_pps_flag is ‘0’, thesubpicture ID of each subpicture may have subpicture index valuesfalling within a range from ‘0’ to ‘sps_num_subpics_minus’.

FIG. 31 illustrates a change in a subpicture ID according to an example.

When a specific subpicture is treated as a picture in an SPS, thesubpicture ID of the specific subpicture in a PPS may be changed.

As illustrated in FIG. 31 , the location of a subpicture in referencepicture N−1 and the location of the subpicture in reference picture N(i.e., the target picture) may not be identical to each other.

Due to the difference in location, when motion compensation using thereference picture of a target subpicture is performed ininter-prediction, the locations of subpictures of the reference pictureshall be derived.

In an embodiment, in order to provide the locations of the subpicturesof the reference picture, the locations of the subpictures of thereference picture may be signaled at the level of at least one of aslice header, a PPS, and a picture header (PH) of the reference picture.

When bidirectional prediction is used for the target subpicture, thelocations of all subpictures both for reference picture list L0 and inreference picture list L1 may be provided.

1) When a specific subpicture is treated and processed in the same wayas a picture, or 2) when one subpicture is encoded and/or decodedindependently of other subpictures, the target subpicture may be forcedto refer to only a reference subpicture having a subpicture ID identicalto that of the target subpicture. Independent encoding and/or decodingmay include at least one of intra-prediction, inter-prediction, andin-loop filtering.

If the target subpicture refers to a reference subpicture having asubpicture ID different from that of the target subpicture, a procedurefor determining whether such reference is to be permitted may beadditionally performed. For this determination, a flag indicatingwhether reference is to be permitted may be signaled. Such a flag may besignaled at the level of at least one of an SPS, a PPS, and a SH.

Method for Determining NAL Unit Type of Rearranged Subpicture

For one or more pictures in a sequence, when the subpicture ID ofsubpicture i of a picture is determined through an SPS, the subpictureID of the subpicture i may be sps_subpic[i]. Here, subpicture i mayindicate a subpicture having a subpicture index of i.

When the target picture is not a first picture in the sequence, thesubpicture ID of the subpicture i of the target picture may bedetermined through a PPS. Here, the subpicture ID of the subpicture i ofthe target picture (i.e., SubpicIdVal[i] of the subpicture i of thetarget picture) may be different from the subpicture ID of thesubpicture i of a previous picture (i.e., SubpicIdVal[i] of thesubpicture i of the previous picture). The previous picture may be apicture that is encoded and/or decoded prior to the target picture inthe order of encoding and/or decoding.

The rearrangement of the subpicture may mean that the subpicture ID ofthe subpicture i of the target picture has changed from the subpictureID of the subpicture i of the previous picture.

When the rearrangement of the subpicture is applied, the NAL unit typeof the slice of the rearranged subpicture in the target picture may beone of values ranging from ‘Instantaneous Decoding Refresh(IDR)_W_Random Access Decodable Leading (RADL)’ to ‘CRA NUT’. When therearrangement of the subpicture is applied, the NAL unit type of theslice of the rearranged subpicture in the target picture may beIDR_W_RADL, IDR_N_LP, or CRA_NUT.

A NAL unit having the NAL unit type of IDR_W_RADL or IDR_N_LP mayindicate a coded slice for an Instantaneous Decoding Refresh (IDR)picture or an IDR subpicture.

A NAL unit having a NAL unit type of CRA_NUT may be a coded slice of aninstantaneous Clean Random Access (CRA) picture or subpicture.

In other words, when rearrangement of a subpicture occurs in the targetpicture in the sequence, the slice in the subpicture of the targetpicture may be a slice in an Intra-Random Access Point (IRAP)subpicture. The IRAP subpicture may be a subpicture for which all VideoCoding Layer (VCL) NAL units have the value of the same NAL unit typeand for which the value of the NAL unit type falls within a range from‘IDR_W_RADL’ to ‘CRA_NUT’.

When subpictures in one picture have values of different NAL unit types,the values of different NAL unit types for the subpictures in onepicture may be signaled through the syntax element“mixed_nalu_types_in_pic_flag” in the PPS.

mixed_nalu_types_in_pic_flag may be information indicating whether mixedNAL units are to be used for the picture.

When the value of mixed_nalu_types_in_pic_flag in the PPS is ‘0’, thevalues of NAL unit types must be identical to each other for all VCL NALunits of the picture.

When the value of mixed_nalu_types_in_pic_flag in the PPS is ‘1’, thepicture may have at least two subpictures, and the VCL NAL units of thepicture may have two or more different NAL unit type values.

Picture in which Mixed NAL Units are Used

A slice header may access information in a PPS through a PPS ID. Apicture header may include the PPS ID.

A slice header may include the syntax element“picture_header_in_slice_header_flag”. Here,picture_header_in_slice_header_flag may be information indicatingwhether a picture header is present in a slice header.

When the value of picture_header_in_slice_header_flag in the sliceheader is ‘1’, the slice header may include the syntax structure of thepicture header.

When the value of picture_header_in_slice_header_flag in the sliceheader is ‘0’, the syntax structure of the picture header may not bepresent in the slice header, and a picture unit for the target pictureshall have the NAL unit of the picture header. Further, in this case,the values of picture_header_in_slice_header_flag of all slices in thetarget picture may be ‘0’.

Therefore, in order to determine the value ofmixed_nalu_types_in_pic_flag, the values of mixed_nalu_types_in_pic_flagin the PPS must be identified through the PPS ID of the picture header.

When the value of mixed_nalu_types_in_pic_flag for the picture is 1, thefollowing conditions 1) and 2) may be applied.

1) A picture may include slices having a NAL unit type of IDR, and oneor more slices of the picture may have NAL unit types corresponding toother values. Here, the following items 1-1) to 1-4) may be applied.

1-1) A certain slice may belong to a certain subpicture, in which thevalue of subpic_treated_as_pic_flag[i] is ‘1’, among the subpictures ofthe target picture.

1-2) A certain slice may not be included in a subpicture which includesa Variable Length Coding/Code (VLC) NAL unit for which a NAL unit typeis not IDR, among the subpictures of the target picture.

1-3) When the NAL unit type of the certain slice of the target pictureis CRA, for all subsequent picture units following the target picture ina decoding order and an output order, reference picture list L0 (e.g.,RefPicList[O]) and reference picture list L1 (e.g., RefPicList[1]) forslices of a certain subpicture may include none of pictures precedingthe target picture in a decoding order in an activation list.

1-4) When the NAL unit type of a certain slice is IDR_W_RADL orIDR_N_LP, for all subsequent picture units following a target picture(e.g., picA) in a decoding order, reference picture list L0 (e.g.,RefPicList[O]) and reference picture list L1 (e.g., RefPicList[1]) forslices of a target subpicture (e.g., subpicA) in the correspondingpicture unit may include none of pictures preceding the target picture(e.g., picA) in a decoding order in an activation list.

2) The case where the value of mixed_nalu_types_in_pic_flag for apicture is ‘1’ may mean that other pictures that refer to the PPSinclude slices having other NAL unit types. For example, these picturesmay be pictures encoded by combining subpicture bitstreams. When thiscombination is performed, the encoding apparatus 100 may performmatching between the structure of the combined bitstream and theparameters of original bitstreams.

FIG. 32 illustrates a part of the structure of a sequence parameter setindicating whether there is syntax related to a reference picture listaccording to an example.

An SPS may include the syntax element “sps_idr_rpl_present_flag”. Thesyntax element “sps_idr_rpl_present_flag” may be information indicatingwhether reference picture list (RPL) syntax elements may be present inthe slice headers of slices for which a NAL unit type is IDR_N_LP orIDR_W_RADL.

When the value of sps_idr_rpl_present_flag is ‘1’, the RPL syntaxelements may be present in the slice headers of the slices for which aNAL unit type is IDR_N_LP or IDR_W_RADL.

When the value of sps_idr_rpl_present_flag is ‘0’, the RPL syntaxelements may not be present in the slice headers of slices for which aNAL unit type is IDR_N_LP or IDR_W_RADL.

FIG. 33 illustrates a part of the structure of a picture parameter setindicating whether there are mixed NAL unit types according to anexample.

A PPS may include the syntax element “mixed_nalu_types_in_pic_flag”.

When the value of mixed_nalu_types_in_pic_flag is ‘1’, each picture thatrefers to the PPS may include one or more VCL NAL units, wherein the NALunit type values of the one or more VCL NAL units may not be identicalto each other. Also, each picture that refers to the PPS may not be anIRAP picture.

When the value of mixed_nalu_types_in_pic_flag is ‘0’, each picture thatrefers to the PPS may include one or more VCL NAL units, wherein the NALunit type values of the one or more VCL NAL units may be identical toeach other.

When the NAL unit type of the slice is an IDR type, a reference picturelist may be configured based on the setting of the syntax elements“sps_idr_rpl_present_flag” and “mixed_nalu_types_in_pic_flag”.

FIG. 34 illustrates a slice header configuring a reference picture listaccording to an embodiment.

In an embodiment, according to the slice header of FIG. 34 , thefollowing schemes may be applied to the configuration of the referencepicture list.

1) A reference picture list may be restrictively configured for asubpicture only when mixed NAL units are used for a picture. As anexample of this restrictive configuration, as illustrated in FIG. 34 , areference picture list may be configured for the subpicture when thevalue of sps_idr_rpl_present_flag in an SPS is ‘1’ and the value ofmixed_nalu_types_in_pic_flag in a PPS is ‘1’.

2) Alternatively, in another example, even if the value ofmixed_nalu_types_in_pic_flag is ‘0’, a reference picture list may beconfigured for a subpicture when the value of sps_idr_rpl_present_flagis ‘1’.

3) Alternatively, in a further example, even if the value ofsps_idr_rpl_present_flag is ‘0’, a reference picture list may beconfigured for a subpicture when the value ofmixed_nalu_types_in_pic_flag is ‘1’.

4) When the values of both sps_idr_rpl_present_flag andmixed_nalu_types_in_pic_flag are ‘0’, a reference picture list for asubpicture may not be configured.

As described above with reference to the above-described embodiments,the configuration of a reference picture list may be adaptivelyperformed based on at least one of the syntax elementssps_idr_rpl_present_flag, rpl_info_in_ph_flag, and NAL unit type-relatedinformation (e.g., mixed_nalu_types_in_pic_flag).

For the purpose of such adaptive performance, the syntax element“rpl_info_in_ph_flag” may be used. “rpl_info_in_ph_flag” may beinformation indicating whether information or syntax related to areference picture list is present in a picture header.

Whether a reference picture list is to be configured in the slice headermay be determined based on at least one of 1) the value ofrpl_info_in_ph_flag, 2) NAL unit type, 3) the value ofsps_idr_rpl_present_flag, and 4) the value ofmixed_nalu_types_in_pic_flag.

For example, the configuration of a reference picture list in the sliceheader may be performed only when the value of rpl_info_in_ph_flag is‘0’ in the slice header.

For example, the configuration of a reference picture list in the sliceheader may be performed when the NAL unit type is none of specifictypes. The specific types may be IDR_W_RADL and IDR_N_LP.

For example, the configuration of a reference picture list in the sliceheader may be performed when the value of sps_idr_rpl_present_flag is‘1’ and the value of mixed_nalu_types_in_pic_flag is ‘1’.

As illustrated in FIG. 34 , the above-described conditions may becombined with each other. The configuration of a reference picture listin the slice header may be performed when condition 1 is true andcondition 2 is true. Condition 1 may be a condition that the value ofrpl_info_in_ph_flag in the slice header be ‘0’. Condition 2 may be acondition that 1) a NAL unit type be none of specific types, or acondition that 2) the values of both sps_idr_rpl_present_flag andmixed_nalu_types_in_pic_flag be ‘1’.

Next, whether an override is to be performed on the number of activated(or active) reference indexes in the slice header may be determined.

Whether an override is to be performed may be determined based on atleast one of 1) the value of rpl_info_in_ph_flag, 2) the NAL unit type,3) the value of sps_idr_rpl_present_flag, 4) the value ofmixed_nalu_types_in_pic_flag, 5) the slice type of each slice, and 6)the number of entries in a reference picture list.

For example, an override may be performed when the value ofrpl_info_in_ph_flag is ‘1’.

For example, an override may be performed when the NAL unit type is noneof the specific types. The specific types may be IDR_W_RADL andIDR_N_LP.

For example, an override may be performed when the value ofsps_idr_rpl_present_flag is ‘1’ and the value ofmixed_nalu_types_in_pic_flag is ‘1’.

For example, an override may be performed when the type of a slice isnot ‘I’ and the value of num_ref_entries[0][RplsIdx[0] ] is greater than‘1’. Alternatively, an override may be performed when the type of aslice is ‘B’ and the value of num_ref_entries[1][RplsIdx[1] ] is greaterthan ‘1’.

num_ref_entries[listIdx][rplsIdx] may specify the number of entries in aref_pic_list_struct(listIdx, rplsIdx) syntax structure.

An override may include signaling of num_ref_idx_active_override_flag.

The syntax element “num_ref_idx_active_override_flag” may be informationindicating whether an override is to be performed on the number ofactivated reference indexes for the reference picture list.

The syntax element “num_ref_idx_active_override_flag” may be informationindicating whether, for a P slice and a B slice, the syntax element“num_ref_idx_active_minus1[0]” is provided, and for the B slice, thesyntax element “num_ref_idx_active_minus1[1]” is provided.

When the value of num_ref_idx_active_override_flag is ‘0’,num_ref_idx_active_minus1[0] and num_ref_idx_active_minus1[1] may not bepresent.

The syntax elements “num_ref_idx_active_minus1[0]” and“num_ref_idx_active_minus1[1]” may be information indicating the numberof activated reference indexes in a specific reference picture list.

The syntax element “num_ref_idx_active_minus1[0]” may indicate thenumber of activated reference indexes−1 for reference picture list L0.

The syntax element “num_ref_idx_active_minus1[1]” may indicate thenumber of activated reference indexes−1 for reference picture list L1.

If the value of num_ref_idx_active_override_flag is ‘1’, the syntaxelement “num_ref_idx_active_minus1[0]” may be present both for the Pslice and the B slice and the syntax element“num_ref_idx_active_minus1[1]” may be present for the B slice when thevalue of num_ref_entries[0][RplsIdx[0] ] is greater than ‘1’.

If the value of num_ref_idx_active_override_flag is ‘1’, signaling ofnum_ref_idx_active_minus1 may be performed when the value ofnum_ref_entries[i][RplsIdx[i] ] is greater than ‘1’. Here, when the typeof a slice is ‘B’, signaling of num_ref_idx_active_minus1[0] andnum_ref_idx_active_minus1[1] may be performed. When the type of a sliceis not ‘B’ (i.e., when the type of the slice is ‘P’), signaling ofnum_ref_idx_active_minus1[0] may be performed. The above-describedmethods for configuring reference picture lists may be applied equallyor similarly to other embodiments, which will be described later.Hereinafter, repeated descriptions may be omitted.

In an embodiment, in the slice header, a reference picture list may beconfigured regardless of (or independently of) sps_idr_rpl_present_flag.A reference picture list may be configured regardless of (orindependently of) rpl_info_in_ph_flag and the NAL unit type-relatedinformation.

The above-described methods for configuring reference picture lists maybe applied equally or similarly to other embodiments, which will bedescribed later. Hereinafter, repeated descriptions may be omitted.

Constraints for reference picture lists configured for the slice of thesubpicture of the target picture may be identical to those of referencepicture lists configured for the target picture.

For example, if the value of sps_idr_rlp_present_flag is ‘1’, when thepicture is an Instantaneous Decoding Refresh (IDR) picture, or even whena subpicture is an IDR subpicture, a reference picture list may beconfigured for the slice.

For example, a reference picture list may be configured even in the casein which a subpicture is rearranged and is then changed to an IRAPsubpicture.

For example, when a target subpicture is a Clean Random Access (CRA)picture, reference picture list L0 (e.g., RefPicList[O]) and referencepicture list L1 (e.g., RefPicList[1]) may include a picture including anIRAP subpicture that precedes the target subpicture in a decoding orderor an output order, and may not include a picture that precedes thepicture including the IRAP subpicture in a decoding order or an outputorder.

For example, when the target subpicture is a Random Access DecodableLeading (RADL) picture, a specific picture for the RADL picture shallnot be an active entry in reference picture list L0 or reference picturelist L1. Here, the specific picture may be 1) a picture including aRandom Access Skipped Leading (RASL) subpicture or 2) a picture thatprecedes a picture including an associated IRAP subpicture in a decodingorder.

In order to use a cross-component adaptive loop filter, signaling may beperformed through the syntax element “sps_ccalf_enabled_flag” defined inan SPS.

The syntax element “sps_ccalf_enabled_flag” may be informationindicating whether the cross-component adaptive loop filter is to beused. When the value of sps_ccalf_enabled_flag is ‘0’, a cross-componentadaptive loop filter may not be used. When the value ofsps_ccalf_enabled flag is ‘1’, a cross-component adaptive loop filtermay be used. When sps_ccalf enabled_flag is not signaled in the SPS orwhen sps_ccalf_enabled_flag is not present in the SPS, the value ofsps_ccalf_enabled_flag may be regarded as ‘0’.

FIG. 35 illustrates the case where locations of subpictures in a pictureare changed according to an embodiment.

In FIG. 35 , an N-th picture, an N+1-th picture, and an N+2-th pictureare illustrated.

Each of the pictures may include subpicture 1, subpicture 2, subpicture3, and subpicture 4. For the illustrated “subpicture n”, “n” may denotethe subpicture ID of the corresponding subpicture.

In FIG. 35 , arrows are present between corresponding subpictures n ofthe two pictures. An arrow from the subpicture of a first picture (i.e.,N-th picture) to the subpicture of a second picture (i.e., N+1-thpicture) may mean that the subpicture at the start point of the arrow isreferred to by the subpicture at the end point of the arrow.

In FIG. 35 , compared to the N-th picture, the locations of subpicture 4and subpicture 2 are exchanged with each other in the N+1-th picture. Inother words, rearrangement of the subpictures may be applied to theN+1-th picture.

When the locations of subpicture 4 and subpicture 2 are exchanged witheach other in the N+1-th picture, the value ofmixed_nalu_types_in_pic_flag in a PPS for the N+2-th picture may be setto ‘1’. Further, the slice type of subpicture 4 of the N+2-th picturemay be one of IDR_W_RADL, IDR_N_LP, and CRA_NUT, and the slice type ofsubpicture 2 may be one of IDR_W_RADL, IDR_N_LP, and CRA_NUT.

In the case where a NAL unit type of a slice in subpicture 4 of theN+2-th picture is IDR, a reference picture list may be configured whenthe value of sps_idr_rpl_present_flag is ‘1’ and the value of mixed_naltypes_in_pic_flag is ‘1’.

After a first slice header of the target picture has been parsed,reference picture marking may be performed.

When a NAL unit type of a slice in the last subpicture (or thesubpicture) of the corresponding picture is IDR, a reference picturelist may be configured.

Here, when the value of mixed_nalu_types_in_pic_flag is set to ‘1’, areference picture list may be configured even if the NAL unit type of acertain slice in the picture is set to IDR.

After decoding performed on all slices in the picture has beencompleted, a decoded target picture may be marked with “used forshort-term reference”.

FIG. 36 illustrates the case where locations of subpictures treated aspictures are changed according to an embodiment.

In FIG. 36 , an N-th picture, an N+1-th picture, and an N+2-th pictureare illustrated.

Each of the pictures may include subpicture 1, subpicture 2, subpicture3, and subpicture 4. For the illustrated “subpicture n”, “n” may denotethe subpicture ID of the corresponding subpicture.

In FIG. 36 , arrows are present between corresponding subpictures n ofthe two pictures. An arrow from the subpicture of a first picture (i.e.,N-th picture) to the subpicture of a second picture (i.e., N+1-thpicture) may mean that the subpicture at the start point of the arrow isreferred to by the subpicture at the end point of the arrow.

In FIG. 36 , compared to the N-th picture, the locations of subpicture 1and subpicture 3 are exchanged with each other in the N+1-th picture. Inother words, rearrangement of the subpictures may be applied to theN+1-th picture.

When the value of subpic_treated_as_pic_flag for the subpicture is setto ‘1’ in an SPS, the subpicture may be treated as a picture.

When the value of mixed_nalu_types_in_pic_flag in a PPS is ‘1’, the NALunit of at least one subpicture of the picture may have the value of aNAL unit type equal to STSA_NUT, RADL_NUT, RASL_NUT, IDR_W_RADL,IDR_N_LP, or CRA_NUT. In contrast, the NAL units all of othersubpictures of the picture may have additional specific values of a NALunit type. The additional specific values may be TRAIL_NUT, RADL_NUT,and RASL_NUT.

A NAL unit having a NAL unit type equal to STSA_NUT may indicate aStep-wise Temporal Sublayer Access (STSA) picture or a STSA subpicture.The STSA picture may be a coded picture for which each VCL NAL unit hasa NAL unit type equal to STSA_NUT. The STSA subpicture may be asubpicture for which each VCL NAL unit has a NAL unit type equal toSTSA_NUT.

A NAL unit having a NAL unit type equal to RADL_NUT may indicate aRandom Access Decodable Leading (RADL) picture or an RADL subpicture.The RADL picture may be a coded picture for which each VCL NAL unit hasa NAL unit type equal to RADL_NUT. The RADL subpicture may be asubpicture for which each VCL NAL unit has a NAL unit type equal toRADL_NUT.

A NAL unit having a NAL unit type equal to RASL_NUT may indicate aRandom Access Skipped Leading (RASL) picture or an RASL subpicture. TheRASL picture may be a coded picture for which at least one VCL NAL unithas a NAL unit type equal to RASL_NUT and other VCL NAL units all have aNAL unit type equal to RASL_NUT or RADL_NUT. The RASL subpicture may bea subpicture for which each VCL NAL unit has a NAL unit type equal toRASL_NUT.

A NAL unit having a NAL unit type equal to IDR_W_RADL or IDR_N_LP mayindicate an Instantaneous Decoding Refresh (IDR) picture or an IDRsubpicture. The IDR picture may be an IRAP picture for which each NALunit has a NAL unit type equal to IDR_W_RADL or IDR_N_LP. The IDRsubpicture may be an IRAP subpicture for which each NAL unit has a NALunit type equal to IDR_W_RADL or IDR_N_LP.

A NAL unit having a NAL unit type equal to CRA_NUT may indicate a CleanRandom Access (CRA) picture or a CRA subpicture. The CRA picture may bean IRAP picture for which each NAL unit has a NAL unit type equal toCRA_NUT. The CRA subpicture may be an IRAP subpicture for which each NALunit has a NAL unit type equal to CRA_NUT.

A NAL unit having a NAL unit type equal to TRAIL_NUT may be a trailingpicture or a trailing subpicture. The trailing picture may be a picturefor which each NAL unit has a NAL unit type equal to TRAIL_NUT. Thetrailing subpicture may be a subpicture for which each NAL unit has aNAL unit type equal to TRAIL_NUT.

As illustrated in FIG. 36 , in the case where the value ofsubpic_treated_as_pic_flag is set to ‘1’ for all subpictures in asequence and the locations of subpictures are changed during encodingand/or decoding of each subpicture of the target picture, a collocatedsubpicture in another picture may not be used as a reference picture.When an area of a first subpicture of a first picture, located in thefirst picture, is identical to an area of a second subpicture of asecond picture, located in the second picture, the first picture and thesecond picture may be subpictures collocated with each other.

When the collocated subpictures cannot be used as reference pictures,the subpicture of the target picture may be processed as an IRAPpicture.

When rearrangement of subpictures is performed, subpictures to whichrearrangement is applied may be processed as IRAP pictures. An IRAPpicture may be a picture for which each VCL NAL unit has a NAL unit typeequal to any one of IDR_W_RADL, IDR_N_LP, and CRA_NUT. In this case,since the subpicture is a part of the picture, the value ofmixed_nalu_types_in_pic_flag in the PPS for the picture shall be ‘1’.

Calculation of a picture order count (POC) for the target picture may beperformed on a first slice of the target picture.

As illustrated in FIG. 36 , the locations of subpictures may be changedin the N+2-th picture. Due to this change, a subpicture at a firstlocation may be changed from subpicture 1 to subpicture 3. Subpicture 3changed to the first location may be processed as an IRAP picture.

Picture types of the subpictures of the N+2-th picture may include bothIRAP and non-IRAP types. In other words, in the picture types of thesubpictures of the N+2-th picture, both IRAP and non-IRAP types may bemixed with each other. Therefore, for decoding of non-IRAP subpictures,a reference picture list may be required. When a reference picture listmay be configured, the reference picture list shall be configured foreach slice in a non-IRAP picture.

Here, when the value of sps_idr_rpl_present_flag in the SPS is ‘1’ andthe value of mixed_nalu_type_in_pic_flag is ‘1’, a reference picturelist may be configured even if the NAL unit type of a slice in thesubpicture is an IDR type.

For a first slice in the N+2-th picture, reference picture marking maybe performed on decoded pictures in a Decoded Picture Buffer (DPB).

After reference picture marking, decoding on each slice may beperformed.

After decoding performed on all slices in the target picture has beencompleted, the decoded target picture may be marked with “used forshort-term reference”. Further, an Inter-Layer Reference Picture (ILRP)in reference picture list 0 or reference picture list 1 may be markedwith “used for short-term reference”.

FIG. 37 illustrates another part of the structure of a picture parameterset indicating whether there are mixed NAL unit types according to anexample.

A description made with reference to FIG. 34 may also be applied to anembodiment which will be described with reference to FIG. 37 . Repeateddescriptions thereof may be omitted.

As illustrated in FIG. 37 , when the value ofmixed_nalu_types_in_pic_flag in a PPS is 1, a reference picture list maybe configured for a subpicture. Here, the configuration of the referencepicture list may be irrelevant to the syntax element“sps_idr_rpl_present_flag” in an SPS. In other words,sps_idr_rplpresent_flag may not be present in the SPS, and a referencepicture list may be configured for the subpicture when the value ofmixed_nalu_types_in_pic_flag in the PPS is ‘1’ even ifsps_idr_rpl_present_flag is not present in the SPS.

Unlike in FIG. 34 , in FIG. 37 , sps_idr_rpl_present_flag was deletedfrom two “if clauses”.

Therefore, in embodiments, 1) a determination of whether a referencepicture list is to be configured in a slice header and 2) adetermination of whether an override is to be performed on the number ofactivated reference indexes in the slice header may be performedregardless of the presence or value of sps_idr_rpl_present_flag. Inother words, 1) a determination of whether a reference picture list isto be configured in the slice header and 2) a determination of whetheran override is to be performed on the number of activated referenceindexes in the slice header may be performed independently ofsps_idr_rpl_present_flag.

Whether a reference picture list is to be configured in the slice headermay be determined based on at least one of 1) the value ofrpl_info_in_ph_flag, 2) the NAL unit type, and 3) the value ofmixed_nalu_types_in_pic_flag.

For example, the configuration of a reference picture list in the sliceheader may be performed only when the value of rpl_info_in_ph_flag is‘0’ in the slice header.

For example, the configuration of a reference picture list in the sliceheader may be performed when the NAL unit type is none of specifictypes. The specific types may be IDR_W_RADL and IDR_N_LP.

For example, the configuration of a reference picture list in the sliceheader may be performed when the value of mixed_nalu_types_in_pic_flagis ‘1’, regardless of the value of sps_idr_rplpresent_flag is ‘1’.

As illustrated in FIG. 36 , the above-described conditions may becombined with each other. The configuration of a reference picture listin the slice header may be performed when condition 1 is true andcondition 2 is true. Condition 1 may be a condition that the value ofrpl_info_in_ph_flag in the slice header be ‘0’. Condition 2 may be acondition that 1) a NAL unit type be none of specific types, or acondition that 2) the value of mixed_nalu_types_in_pic_flag be ‘1’.

Next, whether an override is to be performed on the number of activatedreference indexes in the slice header may be determined.

Whether an override is to be performed may be determined based on atleast one of 1) the value of rpl_info_in_ph_flag, 2) the NAL unit type,3) the value of mixed_nalu_types_in_pic_flag, 4) the slice type of eachslice, and 5) the number of entries in a reference picture list.

For example, an override may be performed when the value ofrpl_info_in_ph_flag is ‘1’.

For example, an override may be performed when the NAL unit type is noneof the specific types. The specific types may be IDR_W_RADL andIDR_N_LP.

For example, an override may be performed when the value ofmixed_nalu_types_in_pic_flag is ‘1’, regardless of the value ofsps_idr_rpl_present_flag is ‘1’.

For example, an override may be performed when the type of a slice isnot ‘I’ and the value of num_ref_entries[0][RplsIdx[0] ] is greater than‘1’. Alternatively, an override may be performed when the type of aslice is ‘B’ and the value of num_ref_entries[1][RplsIdx[1] ] is greaterthan ‘1’.

num_ref_entries[listIdx][rplsIdx] may specify the number of entries in aref_pic_list_struct(listIdx, rplsIdx) syntax structure.

An override may include signaling of num_ref_idx_active_override_flag.

FIG. 38 illustrates an additional case where locations of subpictures ina picture are changed according to an embodiment.

Descriptions of pictures, subpictures, arrows, and rearrangement ofsubpictures, made above with reference to FIG. 35 , may also be appliedto the embodiment which will be described below with reference to FIG.38 . Repeated descriptions thereof may be omitted.

When the locations of subpicture 4 and subpicture 2 are exchanged witheach other in the N+1-th picture, the value ofmixed_nalu_types_in_pic_flag in a PPS for the N+2-th picture may be setto ‘1’. Further, the slice type of subpicture 4 of the N+2-th picturemay be one of IDR_W_RADL, IDR_N_LP, and CRA_NUT, and the slice type ofsubpicture 2 may be one of IDR_W_RADL, IDR_N_LP, and CRA_NUT.

In the case where the NAL unit type of a slice in subpicture 4 of theN+2-th picture is IDR, a reference picture list may be configured whenthe value of mixed_nal_types_in_pic_flag is ‘1’. Here, the configurationof the reference picture list may be irrelevant to the syntax element“sps_idr_rpl_present_flag” in an SPS. In other words,sps_idr_rpl_present_flag may not be present in the SPS, and a referencepicture list may be configured for the subpicture when the value ofmixed_nalu_types_in_pic_flag in the PPS is ‘1’ even ifsps_idr_rpl_present_flag is not present in the SPS.

After a first slice header of the target picture has been parsed,reference picture marking may be performed.

When a NAL unit type of a slice in the last subpicture (or thesubpicture) of the corresponding picture is IDR, a reference picturelist may be configured.

Here, when the value of mixed_nalu_types_in_pic_flag is set to ‘1’, areference picture list may be configured even if the NAL unit type of acertain slice in the picture is set to IDR.

After decoding performed on all slices in the picture has beencompleted, a decoded target picture may be marked with “used forshort-term reference”.

FIG. 39 illustrates the case where locations of subpictures treated aspictures are changed according to an embodiment.

Descriptions of pictures, subpictures, arrows, and rearrangement ofsubpictures, made above with reference to FIG. 36 may also be applied tothe embodiment which will be described below with reference to FIG. 39 .Repeated descriptions thereof may be omitted.

When the value of subpic_treated_as_pic_flag for a subpicture is set to‘1’ in an SPS, the subpicture may be treated as a picture.

When the value of mixed_nalu_types_in_pic_flag in a PPS is ‘1’, asubpicture of a picture may have a NAL unit having a NAL unit type equalto STSA_NUT, RADL_NUT, RASL_NUT, IDR_W_RADL, IDR_N_LP, or CRA_NUT. Incontrast, all of other subpictures of the picture shall have anadditional specific value of the NAL unit type. The additional specificvalue may be TRAIL_NUT, RADL_NUT or RASL_NUT.

Descriptions of STSA_NUT, RADL_NUT, RASL_NUT, IDR_W_RADL, IDR_N_LP, andCRA_NUT, made above with reference to FIG. 36 , may be applied to theembodiment which is described with reference to FIG. 39 . Repeateddescriptions thereof may be omitted.

As illustrated in FIG. 39 , in the case where the value ofsubpic_treated_as_pic_flag is set to ‘1’ for all subpictures in asequence and the locations of subpictures are changed during encodingand/or decoding of each subpicture of the target picture, a collocatedsubpicture in another picture may not be used as a reference picture.When an area of a first subpicture of a first picture, located in thefirst picture, is identical to an area of a second subpicture of asecond picture, located in the second picture, the first picture and thesecond picture may be subpictures collocated with each other.

When the collocated subpictures cannot be used as reference pictures,the subpicture of the target picture may be processed as an IRAPpicture.

When rearrangement of subpictures is performed, subpictures to whichrearrangement is applied may be processed as IRAP pictures. An IRAPpicture may be a picture for which each VCL NAL unit has a NAL unit typeequal to any one of IDR_W_RADL, IDR_N_LP, and CRA_NUT. In this case,since the subpicture is a part of the picture, the value ofmixed_nalu_types_in_pic_flag in the PPS for the picture shall be ‘1’.

Calculation of a picture order count (POC) for the target picture may beperformed on a first slice of the target picture.

As illustrated in FIG. 39 , the locations of subpictures may be changedin the N+2-th picture. Due to this change, a subpicture at a firstlocation may be changed from subpicture 1 to subpicture 3. Subpicture 3changed to the first location may be processed as an IRAP picture.

Picture types of the subpictures of the N+2-th picture may include bothIRAP and non-IRAP types. In other words, in the picture types of thesubpictures of the N+2-th picture, both IRAP and non-IRAP types may bemixed with each other. Therefore, for decoding of non-IRAP subpictures,a reference picture list may be required. When a reference picture listmay be configured, the reference picture list shall be configured foreach slice in a non-IRAP picture.

When the value of mixed_nalu_type_in_pic_flag is ‘1’, a referencepicture list may be configured even if the NAL unit type of the slice inthe subpicture is IDR. Here, the configuration of the reference picturelist may be irrelevant to the syntax element “sps_idr_rpl_present_flag”in an SPS. In other words, sps_idr_rpl_present_flag may not be presentin the SPS, and a reference picture list may be configured for thesubpicture when the value of mixed_nalu_types_in_pic_flag in the PPS is‘1’ even if sps_idr_rpl_present_flag is not present in the SPS.

For a first slice in the N+2-th picture, reference picture marking maybe performed on decoded pictures in a Decoded Picture Buffer (DPB).

After reference picture marking, decoding on each slice may beperformed.

After decoding performed on all slices in the target picture has beencompleted, the decoded target picture may be marked with “used forshort-term reference”. Further, an Inter-Layer Reference Picture (ILRP)in reference picture list 0 or reference picture list 1 may be markedwith “used for short-term reference”.

FIG. 40 illustrates a further part of the structure of a pictureparameter set indicating whether there are mixed NAL unit typesaccording to an example.

A description made with reference to FIG. 34 may also be applied to anembodiment which will be described below with reference to FIG. 40 .Repeated descriptions thereof may be omitted.

When IDR is applied to a single picture other than a subpicture, areference picture list may be configured if the value ofsps_idr_rplpresent_flag in an SPS is ‘1’. Further, even if the value ofsps_idr_rplpresent_flag is ‘0’, when the picture is changed to an IDRpicture through rearrangement of subpictures, the value ofmixed_nalu_types_in_pic_flag is set to ‘1’ in the PPS, and thus areference picture list may be configured for a slice in the subpicture.In other words, when the value of sps_idr_rpl_present_flag is ‘1’ orwhen the value of mixed_nalu_types in_pic_flag is ‘1’, a referencepicture list may be configured.

Therefore, in the embodiments, when the value ofmixed_nalu_types_in_pic_flag is ‘1’, 1) a determination of whether areference picture list is to be configured in a slice header and 2) adetermination of whether an override is to be performed on the number ofactivated reference indexes in the slice header may be performedregardless of the presence or value of sps_idr_rpl_present_flag. Inother words, when the value of mixed_nalu_types_in_pic_flag is ‘1’, 1) adetermination of whether a reference picture list is to be configured inthe slice header and 2) a determination of whether an override is to beperformed on the number of activated reference indexes in the sliceheader may be independent of sps_idr_rpl_present_flag.

FIG. 41 is a flowchart of an encoding method according to an embodiment.

At step 4110, the processing unit 1610 of the encoding apparatus 1600may generate information about a target picture by performing encodingon the target picture.

The information about the target picture may include informationdescribed in connection with the foregoing embodiments and informationincluded in syntax elements illustrated in FIGS. 22 to 40 .

For example, the information about the target picture may includeinformation for a determination of a reference picture list for thetarget picture and information for prediction based on the referencepicture list.

For example, the information about the target picture may be informationused to decode the target picture by the decoding apparatus 1700.Decoding on the target picture may be performed by utilizing theinformation about the target picture.

For example, the information about the target picture may include theabove-described picture information, information about subpictures,information about slices, information about an SPS, information about aPPS, information about a slice header, etc. Further, the informationabout the target picture may include information that is the target tobe signaled.

In an embodiment, encoding of specific information may mean that thespecific information is included in or added to a bitstream.Alternatively, encoding of the specific information may mean signalingof the specific information.

At step 4120, the processing unit 1610 of the encoding apparatus 1600may generate a bitstream including the information about the targetpicture.

At step 4130, the communication unit 1620 of the encoding apparatus 1600may transmit the bitstream including the information about the targetpicture to the decoding apparatus 1700.

Alternatively, the storage unit 1630 of the encoding apparatus 1600 maystore the bitstream including the information about the target picture.

FIG. 42 is a flowchart of a decoding method according to an embodiment.

At step 4210, the processing unit 1710 of the decoding apparatus 1700may receive a bitstream including information about a target picturefrom the encoding apparatus 1600.

Alternatively, the processing unit 1710 of the decoding apparatus 1700may acquire the bitstream including information about the target picturefrom the storage unit 1730.

Alternatively, the processing unit 1710 of the decoding apparatus 1700may acquire a bitstream including information about the target picturefrom a computer-readable storage medium. The computer-readable storagemedium may include the bitstream including the information about thetarget picture.

The processing unit 1710 of the decoding apparatus 1700 may acquire theinformation about the target picture from the bitstream.

At step 4220, the processing unit 1700 of the encoding apparatus 1710may perform decoding on the target picture using the information aboutthe target picture.

For example, the decoding may include a determination of a referencepicture list for the target picture and prediction based on thereference picture list. The information about the target picture mayinclude information described in connection with the foregoingembodiments and information included in the syntax elements illustratedin FIGS. 22 to 40 .

For example, the information about the target picture may include theabove-described picture information, information about subpictures,information about slices, information about an SPS, information about aPPS, information about a slice header, etc. Further, the informationabout the target picture may include information that is the target tobe signaled.

The processing unit 1710 of the decoding apparatus 1700 may extract theinformation about the target picture from the bitstream.

The information about the target picture may be decoded depending on thepriority described in the embodiments described above with reference toFIGS. 22 to 40 .

In an embodiment, decoding of specific information may mean that thespecific information is extracted from the bitstream. Alternatively,decoding of the specific information may mean signaling of the specificinformation.

Provided are a method, an apparatus and a storage medium that performencoding/decoding on an image using a subpicture.

Provided are a method, an apparatus and a storage medium that configurea reference picture list for motion compensation in the case where thelocation of a reference subpicture to be referred to for motioncompensation is different from the location of a target subpicture.

Provided are a method, an apparatus and a storage medium that configurea reference picture list and provide an override for the number ofactivated reference indexes.

The embodiments may be performed using the same method by the encodingapparatus 1600 and by the decoding apparatus 1700. Also, the image maybe encoded/decoded using at least one of the embodiments or at least onecombination thereof.

The order of application of the embodiments may be different from eachother by the encoding apparatus 1600 and the decoding apparatus 1700,and the order of application of the embodiments may be (at leastpartially) identical to each other by the encoding apparatus 1600 andthe decoding apparatus 1700.

The embodiments may be performed for each of a luma signal and a chromasignal, and may be equally performed for the luma signal and the chromasignal.

The form of a block to which the embodiments of the present disclosureare applied may have a square or non-square shape.

The embodiments of the present disclosure may be applied according tothe size of at least one of a target block, a coding block, a predictionblock, a transform block, a current block, a coding unit, a predictionunit, a transform unit, a unit, and a current unit. Here, the size maybe defined as a minimum size and/or a maximum size so that theembodiments are applied, and may be defined as a fixed size at which theembodiments are applied. Further, in the embodiments, a first embodimentmay be applied to a first size, and a second embodiment may be appliedto a second size. That is, the embodiments may be compositely appliedaccording to the size. Further, the embodiments of the presentdisclosure may be applied only to the case where the size is equal to orgreater than the minimum size and is less than or equal to the maximumsize. That is, the embodiments may be applied only to the case where ablock size falls within a certain range.

Further, the embodiments of the present disclosure may be applied onlywhen a condition that the size of a target block is greater than orequal to a minimum size and a condition that the size of the targetblock is less than or equal to a maximum size are satisfied, whereineach of the minimum size and the maximum size may be a size of one ofblocks and units described above in the embodiment. That is, a block tobe a target of the minimum size and a block to be a target of themaximum size may be different from each other. For example, theembodiments of the present disclosure may be applied only when a size ofa target block is greater than a minimum size of a block and less than amaximum size of a block.

For example, the embodiments may be applied only to the case where thesize of a target block is equal to or greater than 8×8. For example, theembodiments may be applied only to the case where the size of a targetblock is equal to or greater than 16×16. For example, the embodimentsmay be applied only to the case where the size of a target block isequal to or greater than 32×32. For example, the embodiments may beapplied only to the case where the size of a target block is equal to orgreater than 64×64. For example, the embodiments may be applied only tothe case where the size of a target block is equal to or greater than128×128. For example, the embodiments may be applied only to the casewhere the size of a target block is 4×4. For example, the embodimentsmay be applied only to the case where the size of a target block is lessthan or equal to 8×8. For example, the embodiments may be applied onlyto the case where the size of a target block is less than or equal to16×16. For example, the embodiments may be applied only to the casewhere the size of a target block is equal to or greater 8×8 and lessthan or equal to 16×16. For example, the embodiments may be applied onlyto the case where the size of a target block is equal to or greater16×16 and less than or equal to 64×64.

The embodiments of the present disclosure may be applied depending on atemporal layer. In order to identify a temporal layer to which theembodiments are applicable, a separate identifier may be signaled, andthe embodiments may be applied to the temporal layer specified by thecorresponding identifier. Here, the identifier may be defined as thelowest (bottom) layer and/or the highest (top) layer to which theembodiments are applicable, and may be defined as being indicating aspecific layer to which the embodiments are applied. Further, a fixedtemporal layer to which the embodiments are applied may also be defined.

For example, the embodiments may be applied only to the case where thetemporal layer of a target image is the lowermost layer. For example,the embodiments may be applied only to the case where the temporal layeridentifier of a target image is equal to or greater than 1. For example,the embodiments may be applied only to the case where the temporal layerof a target image is the highest layer.

A slice type or a tile group type to which the embodiments of thepresent invention to which the embodiments are applied may be defined,and the embodiments of the present invention may be applied depending onthe corresponding slice type or tile group type.

In the above-described embodiments, it may be construed that, during theapplication of specific processing to a specific target, assuming thatspecified conditions may be required and the specific processing isperformed under a specific determination, a specific coding parametermay be replaced with an additional coding parameter when a descriptionhas been made such that whether the specified conditions are satisfiedis determined based on the specific coding parameter, or such that thespecific determination is made based on the specific coding parameter.In other words, it may be considered that a coding parameter thatinfluences the specific condition or the specific determination ismerely exemplary, and it may be understood that, in addition to thespecific coding parameter, a combination of one or more additionalcoding parameters functions as the specific coding parameter.

In the above-described embodiments, although the methods have beendescribed based on flowcharts as a series of steps or units, the presentdisclosure is not limited to the sequence of the steps and some stepsmay be performed in a sequence different from that of the describedsteps or simultaneously with other steps. Further, those skilled in theart will understand that the steps shown in the flowchart are notexclusive and may further include other steps, or that one or more stepsin the flowchart may be deleted without departing from the scope of thedisclosure.

The above-described embodiments include examples in various aspects.Although all possible combinations for indicating various aspects cannotbe described, those skilled in the art will appreciate that othercombinations are possible in addition to explicitly describedcombinations. Therefore, it should be understood that the presentdisclosure includes other replacements, changes, and modificationsbelonging to the scope of the accompanying claims.

The above-described embodiments according to the present disclosure maybe implemented as a program that can be executed by various computermeans and may be recorded on a computer-readable storage medium. Thecomputer-readable storage medium may include program instructions, datafiles, and data structures, either solely or in combination. Programinstructions recorded on the storage medium may have been speciallydesigned and configured for the present disclosure, or may be known toor available to those who have ordinary knowledge in the field ofcomputer software.

A computer-readable storage medium may include information used in theembodiments of the present disclosure. For example, thecomputer-readable storage medium may include a bitstream, and thebitstream may contain the information described above in the embodimentsof the present disclosure.

The computer-readable storage medium may include a non-transitorycomputer-readable medium.

Examples of the computer-readable storage medium include all types ofhardware devices specially configured to record and execute programinstructions, such as magnetic media, such as a hard disk, a floppydisk, and magnetic tape, optical media, such as compact disk (CD)-ROMand a digital versatile disk (DVD), magneto-optical media, such as afloptical disk, ROM, RAM, and flash memory. Examples of the programinstructions include machine code, such as code created by a compiler,and high-level language code executable by a computer using aninterpreter. The hardware devices may be configured to operate as one ormore software modules in order to perform the operation of the presentdisclosure, and vice versa.

As described above, although the present disclosure has been describedbased on specific details such as detailed components and a limitednumber of embodiments and drawings, those are merely provided for easyunderstanding of the entire disclosure, the present disclosure is notlimited to those embodiments, and those skilled in the art will practicevarious changes and modifications from the above description.

Accordingly, it should be noted that the spirit of the presentembodiments is not limited to the above-described embodiments, and theaccompanying claims and equivalents and modifications thereof fallwithin the scope of the present disclosure.

What is claimed is:
 1. A decoding method, comprising: acquiringinformation about a target picture from a bitstream; and performingdecoding on the target picture using the information about the targetpicture, wherein the decoding comprises a determination of a referencepicture list for the target picture and a prediction based on thereference picture list, and wherein the information about the targetpicture includes a slice header for a slice of the target picture. 2.The decoding method of claim 1, wherein the determination of thereference picture list in the slice header is performed when a NetworkAbstraction Layer (NAL) unit type of the slice is not a specific type.3. The decoding method of claim 1, wherein: the bitstream includes apicture header for the target picture, and the determination of thereference picture list in the slice header is performed when informationrelated to the reference picture list is not present in the pictureheader for the target picture.
 4. The decoding method of claim 1,wherein: the bitstream includes first information, the decoding includesa determination of whether an override is to be performed on a number ofactivated reference indexes in the slice header, the determination ofwhether the override is to be performed is performed regardless of avalue of the first information, and the first information is informationindicating whether reference picture list syntax elements are capable ofbeing present in slice headers of slices for which a NAL unit type is aspecific type.
 5. The decoding method of claim 4, wherein: the bitstreamfurther includes second information, the override includes decoding onthe second information, and the second information is informationindicating whether an override is to be performed on the number ofactivated reference indexes for the reference picture list.
 6. Thedecoding method of claim 5, wherein, when the second informationindicates that the override is to be performed, the informationindicating the number of activated reference indexes for the referencepicture list is decoded.
 7. The decoding method of claim 6, wherein thespecific type is an Instantaneous Decoding Refresh (IDR)_N_LP orIDR_W_Random Access Decodable Leading (RADL) type.
 8. An encodingmethod, comprising: generating information about a target picture byperforming encoding on the target picture; and generating a bitstreamincluding the information about the target picture, wherein theinformation about the target picture includes information for adetermination of a reference picture list for the target picture andinformation for a prediction based on the reference picture list, andwherein the information about the target picture includes a slice headerfor a slice of the target picture.
 9. The encoding method of claim 8,wherein the determination of the reference picture list in the sliceheader is performed when a Network Abstraction Layer (NAL) unit type ofthe slice is not a specific type.
 10. The encoding method of claim 8,wherein: the bitstream includes a picture header for the target picture,and the determination of the reference picture list in the slice headeris performed when information related to the reference picture list isnot present in the picture header.
 11. The encoding method of claim 8,wherein: the bitstream includes first information, and the informationabout the target picture further includes information required todetermine whether an override is to be performed on a number ofactivated reference indexes in the slice header, the determination ofwhether the override is to be performed is performed regardless of avalue of the first information, and the first information is informationindicating whether reference picture list syntax elements are capable ofbeing present in slice headers of slices for which a NAL unit type is aspecific type.
 12. The encoding method of claim 11, wherein: thebitstream further includes second information, and the secondinformation is information indicating whether an override is to beperformed on the number of activated reference indexes for the referencepicture list.
 13. The encoding method of claim 12, wherein, when thesecond information indicates that the override is to be performed, theinformation indicating the number of activated reference indexes for thereference picture list is encoded.
 14. The encoding method of claim 13,wherein the specific type is an Instantaneous Decoding Refresh(IDR)_N_LP or IDR_W_Random Access Decodable Leading (RADL) type.
 15. Anon-transitory computer-readable storage medium storing a bitstream,wherein: the bitstream includes information about a target picture, theinformation about the target picture is acquired from the bitstream,decoding on the target picture is performed using the information aboutthe target picture, the decoding includes a determination of a referencepicture list for the target picture and a prediction based on thereference picture list, and the information about the target pictureincludes a slice header for a slice of the target picture.
 16. Thenon-transitory computer-readable storage medium of claim 15, wherein thedetermination of the reference picture list in the slice header isperformed when a NAL unit type of the slice is not a specific type. 17.The non-transitory computer-readable storage medium of claim 15,wherein: the bitstream includes a picture header for the target picture,and the determination of the reference picture list in the slice headeris performed when information related to the reference picture list isnot present in the picture header of the target picture.
 18. Thenon-transitory computer-readable storage medium of claim 15, wherein:the bitstream includes first information, the decoding includes adetermination of whether an override is to be performed on a number ofactivated reference indexes in the slice header, the determination ofwhether the override is to be performed is performed regardless of avalue of the first information, and the first information is informationindicating whether reference picture list syntax elements are capable ofbeing present in slice headers of slices for which a NAL unit type is aspecific type.
 19. The non-transitory computer-readable storage mediumof claim 18, wherein: the bitstream further includes second information,the override includes decoding on the second information, and the secondinformation is information indicating whether an override is to beperformed on the number of activated reference indexes for the referencepicture list.
 20. The non-transitory computer-readable storage medium ofclaim 19, wherein, when the second information indicates that theoverride is to be performed, the information indicating the number ofactivated reference indexes for the reference picture list is decoded.